Wood is good, but I've found that 1/8" masonite is another backing product to consider. Especially, when you are planning on covering it with fabric or vinyl. It's cheap,easy to cut,readily available at lumber supply stores and its most useful characteristic is that it can be permanently curved for headliners and other non-flat applications.
By wetting the back (rough) side, weight can be progressively applied to the area that needs to be curved and overnight the curve will be permanent. I just place a WET rag over the area to be curved....use a C-clamp with weights to draw the curve into shape. I usually support the area to remain flat on a workbench with some weight to hold it in place and hang the area to form over the edge. The key is to add weight slowly, keep the rag WET, and a 90 degree permanent bend can be achieved.
I usually buy the flimsy, ill-fitting repo healiners/door/kick panels to use as a pattern....add or subtract as they never really totally fit. Using my "adjusted" cardboard repo as a pattern, cut out the masonite, and form the part.
I've added a couple pictures to my webshots, entitled "New Album" that shows a panel being curved. Hope this helps!
