Here is a link to a rear belly pan I built a couple of years ago for an Echo.
http://ecomodder.com/forum/showthrea...echo-7380.html
The rear of that car has very poor attachment points so I built a simple frame with thin aluminum extrusions that I purchased at Lowes. These thin extrusions can be bent a bit into gentle curves and can be pop riveted or screwed together to form a basic structure for coroplast.