This is a way of producing small plastic mouldings without the need for any vac-forming.
The first stage of the process is to make a hardwood plug, the better the surface finish the better the final moulding will be. If you’re being pedantic make it smaller by the thickness of the plastic, but knowing what the finished thickness of the plastic will be after moulding would certainly tax my brain past the limit!
Then cut a corresponding hole in some plywood. The clearance between the plug and the hole should be about the thickness of the plastic you intend to use but it isn’t that critical.
Cut your plastic leaving plenty of spare and position it centrally over the hole; I find it helpful to pin the plastic in place to stop it curling when the heat is applied.
Heat the plastic using a heat gun; keep the heat gun moving to get as even a spread of heat as possible over the entire area of the plastic then quickly plunge the plug through the hole to the required depth.
Here are 2 tank sumps for my SE5a, one roughly trimmed.
Getting the right amount of heat takes a bit of trial and error and you will almost certainly have a few failures before you get it right but the plastic can be any thermo plastic from pop bottles to ice-cream containers so it costs nothing to experiment.