Polystyrene (seriously)

It's cheap and bubbly - so it's not nice to work with unless you use some hot-cutting methods (share any tips)
We haven't used it under full infusion, but we are planning to use if with a wet-layup under compression molding.

It is a bit springy, so final cure pressures should be low & it's why we wouldn't use it for full infusion (it would probably deform while wetting out the cloth under full pressure).

But for a CNC enclosure ..... Let's just say we don't like throwing anything away and some of the great discoveries can be/have been made by accident !!!