I purchased a Wetsounds 10" subwoofer to replace the factory Rockford Fosgate sub that was over 10 years old. The factory enclosure was HDPE and not at all sealed and much too small in air space. The sub specs said it needed 1.3 Cu ft. I measured the under dash area at the back (against the heater) and at the front. For simplicity and because I only needed 1.3 cuft I decided to make a more standard shaped box and then use a faceplate to fill out the front area. I also wanted to integrate heater tubes for the driver and passengers in to the faceplate.
I built the box from 3/4" MDF and then fiberglass resin coated the exterior and interior. The outer was sanded and then painted gloss black with acrylic tractor paint.
I designed the box to actually sit on a 3/4" HDPE panel to elevate it off the floor and then the front face extends 5/8" below so that it does not quite sit on the floor. This prevents any concerns with eventual moisture absorption or wear of the waterproof coatings applied.
I designed the entire thing in 3d CAD so that I could check the inner volume. I cut samples from cardboard for test fitting before cutting any wood.
I designed some EVA foam to go on the front panel to dress it up and match the flooring project I also completed.