i'm no expert, so do your own research to make sure i'm not misinforming you...
buuuut what i'd do 9in this situation is similar to what the manufacturer does
i'd take some half inch MDF or something similar
make cardboard mockup of the floor and places that you want to be flat
cut those parts out of the wood and line your trunk
for all the other really curvy parts, lay down painters tape, then some fiberglass matting and resin. let dry and you have the exact forms of the trunk
sand, paint, or cover in carpet.