If you can make an aero cap rather than a square cap. That'll be one of your biggest gains for reducing highway speed engine load.
I'm sure you've seen lots of examples of DIY aero caps, from pro-quality to quick-n-dirty.
EG:
http://ecomodder.com/forum/showthrea...cap-30843.html
If a tapered topper doesn't work for you, then look at making tapered flaps for the rear.
Have you run down the rest of the aero mods list? Wheel covers? Rear wheel fender skirts? Belly pan?
And.. welcome to the forum!