Here is a boat tail tested for a GM SUV.It was good for a 26% drag reduction (about 13% mpg).Kinda long.
You wouldn't necessarily want the length around town,but for open road cruising,any length would be welcome.
I looked at a JEEP Wrangler Rubicon the other day.It has a very stout bumper-mounted,swing-away spare-tire mount which would be ideal for a swing-away boat tail.
A tail could be fabricated and mounted to the JEEP-like mount,which would allow access to the regular rear doors.
