Member called Vekke had number of impressive mods done to his VW Lupo and from pictures he published all started in backyard and kitchen. Look up his posts and webpage.
There ia also technique fiberglassing over foam, that looks time and cost effective. When I am on my PC, i will provide a link. Edit: found it here:
https://rqriley.com/fiberglass-over-foam/
Hth