Sundown SD-2 8's would solve your problem as they only need a 4.75 inch mounting depth and they can be mounted right against the inside of the box. They also only require .25 to .5 cubic feet each and they are rated at 300RMS each. You could get a couple of these and downfire them. Just leave at least an inch from the top of the surround so they don't hit the floor.
Sundown SD-2 8's would solve your problem as they only need a 4.75 inch mounting depth and they can be mounted right against the inside of the box. They also only require .25 to .5 cubic feet each and they are rated at 300RMS each. You could get a couple of these and downfire them. Just leave at least an inch from the top of the surround so they don't hit the floor.
Thx, but the depth isn't the issue, it's the height. If the sub's overall diameter exceeds 8", that's where I have a slight issue now. All good, people have suggested plenty of good options here, I appreciate the input provided so far.