Fill the cavity with Beanie Babies and cover it with the plywood. Use the carpet as a template.Maybe cover the floor with 1/2" plywood?
Most wood is overpriced right nowJust wish MDF wasn't so expensive.
I too generally use the factory carpet to make my templates. It's also a good idea to put layer of butyl damping material under the MDF too, prevents vibration chattering later on down the line.I actually like working with MDF. It's easy to cut and shape. Plus it takes glue very well.