This is what I'd do (I've done this technique when I built full size airplane parts) I would draw the plans to full scale and print them in sections and tape them together, then out in plywood to buck, once your buck is done, you can lay your fiberglass over the buck and make your mold. I use 3d software for the design and dynamics, and always print them out.

I've been building and flying full size airplanes since 2005

Take a look at Chris Runge and Runge Cars. Check out his Facebook and Instagram pages if you have access. Even Youtube. You can see the wooden body bucks he built and actual cars after. Fascinating example.

You should build a real size model yourself using measuring tools.

make some kind of armature that would be able to withstand the weight of the plastic then i've got a genius idea .
cut each panel so it can be printed and on the side of each panel put holes and on the adjacent panel make some cylindrical shapes that would fit into these said holes
i'm currently doing it and printing some kind of 1/5 life size model of a car rn
then just use a glue gun , sand and lay some fiberglass on top of it