I would use MDF (stronger and easier to work with than standard particle board. Use 1/4" for side panels and probably 1/2" - 5/8" for anything that will be supporting weight. There was this stuff we used to use back in my car install days called bend-a-board. It looked like plywood and was maybe 1/8-1/4" thick. You could bend it pretty easily and then use some kind of framing to keep the shape. The other way you can go is fiberglass, especially if you want to make something with curves in it. That's quite a bit more work, however.
I would try 1/4" MDF for the underside of the trunk lid. You can cut your basic shape and use some coarse sandpaper to shape the edges.
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