I'm just going to use a 10" self-amplified bazooka tube. For a long time I ran a 5 channel alpine amp/Rockford separates in the doors/Infinity Kappas in the dash/4x10 Pioneers & a single 8" Punch in a very small box on the trunk ledge.... But I ditched the Kappas, too much "highs", swapped the 4x10 pioneers for some Boston 6x9's, ditched the amp & small sub. Going to drive the mids/highs off the new deck & the 10" bazooka in place of the small 8" I had. Some like the Tubes, some don't. I have an 8" in my truck & a 10" in the vette and really like the simplicity of them. Going to build a board that will "enclose" that section behind the seat where the trunk floor goes up and cover it with some dynamat on one side & carpet on the other to hide the tube.
If you don't want to go the tube route, you can always build a box out of MDF. It's a pretty square size you'd need and you could build it to the exact size that the sub you pick requires. Not too difficult, brace it, caulk it and cover with some carpet. Most subs will tell you the cubic feet requirement whether you're going sealed/ported or bandpass.