How do i make a speaker box for my hatchback??

I attached a photo of mine (sorry I would take it apart and show you but my mustang is still in storage). Its basically a square box made of MDF carpeted on top with black carpet and then a piece of plywood with the stock carpeting. It's a 15" sub so it takes up allot of room and the box goes right in where the spare tire is supposed to go, I like the design but I blew a tire once and had to call a tow truck lol. It looks good and its a very cheap design I've also seen people use a mixture of fiber glass and MDF to make the sub box either way works well.