The rear box cavity will need a diffuser and the diffuser will need at least a partial under tray (belly pan). So I am thinking of designs.
The first four pages of this diffuser build (and one later post with photos of the prototype) are useful:
http://ecomodder.com/forum/showthrea...nel-15189.html. I want to imitate his use of togglebolts in the existing subframe, aluminum c-channels(?), and folded coroplast on the diffuser.
Here is an image of a recent Civic hybrid's undertray. I like the fact that they leave space for things to heat to escape and for service access. I'm also thinking of oil drip from my ancient engine:
And here is my first effort at "drawing" a prototype:
dark green = vertical line, curved inward toward the trunk lid
orange = vertical section follows tumblehome, curves to spoiler & trunklid
light green = flat spoiler
The goal is to allow flow to "skip" the dead air around the tail lights, create cleaner separation and a smaller wake, and therefore as I understand it produce less drag. I know any benefit will be smaller than a boat tail. I don't want a boattail because of the loss of window and trunk functionality and the increased fabrication difficulty.