Hitch mounted boat tail idea, yes another one.
 

The other day I located a company here in Australia that does hitch mounted carriers, fully compliant and legal throughout the country, Link below, It's on the gallery page and the third picture with the large 4x4 wagon with the dresser/cabinet on the carrier is what I imagine my vehicle would look like:
HitchMate Cargo Carriers Gallery, Motorcycle Carriers Gallery

So if I was to create a "Tool Box" on the back, that just happened to conform with the stream lining template in the design, throw a couple of light tools in there & no more questions from officialdom, it can carry up to 160kg which is more than enough for my purposes, and I might actually carry something with it on occassion.
It comes back about 2 feet from the body, which would give me about 3 feet horizontally from the roof edge, this should be ample to get a significant Kamback/semi Boat Tail structure happening, I could also longer term close the gap between the vehicle and carry box with appropriately shaped foam strip so that it would be completely smooth in airflow transition.

I will start searching for all relevant threads tonight, but am fishing for any comments before I part with $$$$.

Thanks in Advance

Sounds like a good plan. I wonder how much it will wiggle side to side due to the loose tolerances in hitch design. Maybe you'll have to do some sort of straps around the truck's rear hatch to secure the boat tail. Go for it, and report back!
Make sure you have a belly pan first though, as it will help the boat tail function properly. You're trying to move the point of flow separation back a few feet, and if the air is already separated it does no good.

With a box shaped rear end you'll want a zero degree angle on the rear of the belly pan. Completely horizontal- any upward slope will increase drag comparatively. I'm not sure how far back this applies so you may want to ask Aerohead and study Basjoos' AeroCivic.

Already planning some strapping, have some trampoline springs which I am thinking will be good to take out the slack, also planning some tension points to attach to the gutters on the roof on either side, this should hold it firm but still allow a bit of give.

Full belly pan is a bit of an issue as there is quite a bit of movement in the solid axles when in the rough stuff, but may be able to look at flattening out the rear from the axle back to give a more tidy entry. Some of the things I've read indicated around 2 degrees on the rear belly/diffuser as an optimum to avoid drag.

Lots of reading to do.

I'll spend the week reading everything I can to evaluate if this is a viable prospect in terms of $$$ and payback, but thanks for the positive input.

Edit: Just did some measurements and the axle itself is 12" off the ground, the diff is about 8", going back the tank guard angles upwards from the axle to the bottom of the towbar to a 16" clearance, that's a rise of 4" over about 36", angle about 6 degrees, but I imagine with the overall height I don't expect too many issues with downforce The unit I'm looking at will be about 5" above the tow hitch, this could be done with a tapered rise by attatching a flat panel to the base of the unit which would give about 5" over the 24" depth of the unit. I feel that I need the additional height clearance for dips as I do live in a rural area and like to get out in the scrub regularly.
Anyway will start searching the site for more info.

Did some more measurements, bit rough, just for a guesstimate.
Current rear dimensions and area in inches - 72 wide base, 48 wide top x 60 high, by my calculations = 3600"sq
at about 14 degrees on top at 36" extension is 9" drop in height, 14 degrees at each side at about 24" come in 6" each.
New rear area would be 60 wide base, 36wide top x 51 high, which comes to 2448"sq.
As a percentage that is 32%, so going by the general rules I could expect 16% FE improvement, providing it is done right.
Particularly if I skirt the rear wheels and taper in from the skirts in the lower section with other trimmings I could hope to crack the 15% mark.
Most of my travel is highway & higher speeds generally.
Not just a pipe dream I hope, will go and do some Payback figures based on a 15% improvement.
Edit 1 : Assumption error, just remembered read somewhere around 30% drag is from the back & overall aerodynamic drag is 65% of fuel consumption at higher speeds, this would mean a 30% reduction in the rear is only 6% improvement in FE.
Edit 2 : Found calculation error in surface area, corrected above, doesn't change much.

Hope there is better opinions out there, please comment on the numbers, what's the likely improvement in fuel economy if rear drag is reduced 30%?

sides and top are pretty conservative at 14 degrees. Consider as much as 20 degrees.

Bottom of the carrier I would not go steeper then 10 degrees - dirty air under the car won't flow well along the bottom.

At the very back of the carrier, turn the edges in like the back of the car is - don't just chop them.

The longer you are willing to make the carrier, the more gains you will see. And even the 3 feet you mention can be a BIG part of changing the surface area on the back of the truck.

This is Another project I'm going to be building in the next few weeks for my Suburban.

I think 30 percent is pretty conservative gain - 50 percent I think is possible.

Why do you say this? It seems that a sharp edge would facilitate clean flow detachment.

delta-Cd/delta mpg
 

On the CRX,a 12" tail and rest of 'kit' got me a 16.5% increase in mpg.
On the VW bus,an 18" tail and other mods got me up to 29.6% mpg.
24" of boat tail is a serious mod
Walter Lay was getting Cd 0.20-21 with 35% of (Template) tail.I think you'll be amazed.

A few things to consider and to make it more practical. http://www.youtube.com/watch?v=5O1KROgn_fo

Sounding a lot more positive,
Boatsapper, yes have been thinking about alternatives to maintain access to rear of vehicle.
Aerohead, that's the sort of numbers I like to hear, that is amazing, if I can get 10-15% fuel economy improvements, I'm sure I would blow the first tanks savings in a drunken stupor.
Drmiller, I have redone the numbers with underside taper at 10 degrees and top and sides at 18 degrees and that rounds of to about a 50% reduction in rear surface area. As Sven7 queried, I also thought the trailing edge should be a sharp cut off, did you mean moreso about following the body lines and curving the side and top panels as they come in?
I'm almost thinking it may be easiest to just get a massive block of styrofoam and shape it appropriately, then put a skin on it. Put some doors on the rear and just have the smaller dimension as the cargo area.
Will go and have another read about the aerocivic and other boattail designs.

If you look at the AeroCivic, he built a structure out of aluminum flat bars. Here's how I intend to make mine. You can do a pretty simple structure if you cross brace it.

Red: bars
Blue: cross bracing wires
Green: skin
Yellow: truck

http://i44.tinypic.com/2ywaj9v.jpg

(Forgot to make side-to-side braces)