Try on a roller if you have one, ive rolled thick stiff cardboard for mock ups at work and it works great
doesnt have a ton of memory, i think the fibers or whatever seperate/stretch inside so usually first light pass thru i get the beginnings of a curve
just my opinion, if you lived closer i'd say come on by and use mine
id bet if you had a "cookie" roller , or 2 inch diameter rod of some sort, you could lie the cardboard part to be rolled on top of a piece of carpet or stiff foam, then place a couple strips under the carpet, leaving the middle as the low point and gently work the middle part with the roller adjusting the strips closer or further to the center to control the curve , a little heat via heat gun or even blow dryer never hurts to get the molecules moving a bit inside, sounds ghetto but just a thought
I didn't think of the roller trick (a good suggestion!) but to install mine, I installed the top edge first, then sort of scooted or pushed the lower end into place while allowing the cardboard to bow out slightly. I did this with the trunk mat pulled back, of course. Oh, and the prestamped holes for the screws don't line up. *sigh*
I made a score line on the cardboard so they would bend easily and stay that way, then I covered them with fake leather material that I bought at a fabric store for a couple bucks. Looks a lot better than the plain cardboard.
(The prop rod is no longer in use - I installed a gas shock so my trunk will stay open after the addition of a spoiler) I also covered the trunk latch with the fake leather. Not original looking, but nice and clean.