For a tonneau cover, I think ABS would be your best choice. It's not much more expensive than coroplast and it will take allot more abuse with a good frame supporting it. You can get it in a large range of thicknesses too, allowing you to adjust the amount of support you plan to build into the frame. Unfortunately you color choices at most places are going to be white or black. I'm not certain, but I think it's paintable however.

well i plan to paint it black anyways:p yea i was thinking ABS, resiliant to cracking, temperature safe, somewhat impact resistant (well hard enough that if somehitng does fall on it it would make a big enough shock to set off the alarm) im betting the place would know the most about plastics and what thicknesses would work best.

now if they can get it in large sheets...:D

i will have to look up some local plastics place( i know of one but dunno the name)

I think 4'x8' is the largest you are going to find, so no matter what size bed you have, you're going to end up with a visible seam somewhere - unless you do something like cover it with UV resistant vinyl.

No matter what you use, just remember that anything that doesn't go over the bed rails is going to leak water into the bed. Be sure to provide somewhere for it to go unless you want your stuff getting wet.

yea my bed is 5X6 and they dont make solid steel sheets like that. i know 4X8 they do tho. who knows maybe plastic is different.

which way would look better to make a seam? lenghtwise(from nose of truck to tailgate) or 1 or 2 sections going the width?(i will try to make it not as visible but i know its gonna happen)

but yea i plan to have it sit flush on the bed rails with a strip of adhesive window weather stripping between the steel frame of the cover and bed(prevent scratches and noise). i also plan to run a bead of silicone caulk between the layers of steel (keep water from gettign between the layers and rusting out and make it waterproof). prolly isnt going to be 100% waterproof but it will pobably be good enough so it doesnt soak whatevers inside.:D

Most recycling centers will have Coroplast signage year round free for the taking.

I was thinking about useing Coroplast for the body of my CitiCar, but I would need to shape some of it, and glue seams, anyone know what kind of plastic it is made out of? PVC or something like that I would assume.
Any ideas on how hard it is to heat and shape? or does it just wrinkle...

Political Signs
 

You could remove the signs that are posted illegally on the sides of the road. Many of them are Coroplast. We pay city employees to remove them!!!

The Obama signs work best.

Coroplast is made of polyethylene(PE). PE doesn't glue well, but can be welded if you're careful with the heat. I've heated and put bends into Coroplast, easiest to bend in the direction of the corrugations, but the corrugations tend to collapse if you put more than a slight curve to it. Works fine for 90 degree bends, that's how I did my side skirts.