My car is 41 Willys (fiberglas). They come with nothing inside at all. These cars ar also all round with almost nothing straight. A "splash" would have been nice when the car was being built but not now after painting. I made 7 roof bows out of 3/4" premium oak planks. I also build giant scale rc warbirds (model airplanes) so working with wood is not real problem. I spliced most of them at least once using both epoxy and Good old Elmers woodworking glue. This was also an economy process for the same reason you listed. Oak cuts very nice and smells nice too, haha
I did make endless patterns both across the body and down the length. I finally settled on 1/8 in welding wire spliced together. A little duct tape and oak spacers positioned the wire stringers so I could make good posterboard patterns for the oak. I used a FatMan laser level to give me accurate lines to work to. I traced these with permanent marker onto the interior of the body. I installed the oak frame or bows with silicone sealer rather than resin as the resin can sistort the body. (found this out the hard way) From there i "upholstered the oak frame with .006 plastic sheet between each bow. I numbered each of these and cut fabric pieces to match and sew them together. I stapled the assembly to the bows starting from the front. It was just like installing a carpet only upside down.
Considering it was my first upholstery project I happy with it.