question for those with experience

lets say i wanted to make a template or mold out of cardboard for like a box, or a custom dash pod or something like that. ideally id like to pull out the map pocket and cup holders in my second gen, creating a large crater so that i can design a new layout with cardboard and then somehow mold a new panel into that space with 2 cupholders and a tray, maybe a wireless charger (i realize i may have to mold over wood or something after creating my template on cardboard)

is there a material that i can pile on top of that mold and then eventually sand down and finish or possibly cover with foam and leather? literally like a sandable clay

some sort of bondo maybe?

kinda like the guys that make crazy custom car audio and subwoofer enclosures although id rather not use fiberglass.

previously i have used tubes and tubes of JB Weld plasticweld. i have filled holes in dashes, made custom switch pods, etc by molding it like clay and sanding it down, but i feel like theres a legit way, im just very bad at explaining this. (also i would need may tubes for this application)

anyone that can point me in the right direction of the right material so that i can learn how to use it.

thanks!!!!!!!