If you are not handy or have a skill saw, take the back off, take it to Lowes, have a guy there cut you a piece of 3/4 plywood that is about 4 or 5 inches off the top. You will have to drill holes in the perimeter of the new board, but it will get you a partial back.
I have an Egnater Tourmaster oversized 2x12. It has that removable panel in the rear, but it has a metal baffle to keep it from 'breathing' too much out the back.
There are also the front ported 2x12 cabs that are extremely popular now, Port City, and then the 1x12 Thieles.
If you have the coin, contacting Boogafunk for a 1x12 Thiele box may work, you can put your speaker in it, go for a second one in time. I heard and almost not believed how good a mini stack of 1x12 Thieles sounds. Way bigger that you would think 2 little 1x12 cabs will sound. Sounds better or bigger/wider than my 2x12 cab. That is the route I am going to go. Just have not decided on the speakers to go in there.
Peace...
Softail