I'm building a prototype electronic device and need to construct a
case for the prototype. The approximated dimensions are 13" x 19" x
3", and it needs to have cooling slots in the bottom for fan driven
airflow.

I'd like as light-weight, but strong material as possible.

My one thought was to recycle PET soda bottles and develop a home-made
pour/compression mold, but I'm not certain that's the practical way.

Or, perhaps using a pourable resin (polyester or epoxy?)

Finally, I've considered a shop-build vacuum forming table, but I'm
concerned about forming the support ribs on the inside of the bottom
case half.

Any suggestions would be welcome.