Gonna be hard work to make it look ok, so good luck with that. Presumably you've trawled the breakers.
Some thoughts.
I think MDF may be a bit heavy. My thoughts would be hardboard, with suitable stiffening underneath, or thin plywood, again with some stiffening.
Make a template from a large cardboard box first (then stiffen that and use it?). You'll need something to buffer it underneath where it rests on the supports, or it will drone and rattle, especially as the sub makes it vibrate.
Cover with vinyl, or better, industrial carpet if you can find some, or seat cover material. Some padding would probably help it not to vibrate.