[JethroBodine]

My OPINION would be to go with option 3 and heat form some lexan and make a flat closure on the back side, paint-match the upper part and leave the rear clear.

If you do use spray foam, let it sit about a week before covering it with anything( even paint) as this will give it time to FULLY cure and won't bubble/warp like my tail is doing right now because I rushed to get it semi finished before the SMCC Rally this last weekend. After a week you can use fiberglass resin to make it tough as nails, and then sand to shape as you see fit.

Coroplast is fun to work with and can be made to do some funky things as some of the artists around here have accomplished with a little creative engineering. Closing the end with lexan will give you the full benefit of view and solid rear face.

Hope that muddies the waters even further and I can't wait to see what you come up with.