Okay, made a slight mistake by purchasing a sub before building the custom enclosure (picture attached, not mine exactly, but very similar). Now I'm limited as far as mounting depth and air space.

It's to be a downfiring sub, approximately 3 inches from the floor. No control over that dimension. The fifth channel of my amp will push 200W RMS.

The top of the box is angled from 6.25" deep to 5.125" deep. I could probably go with a 5.25" deep sub, as the magnet tapers to the back of the sub. The enclosure, (I'm going from memory on the dimensions), is around .4 cf, without the sub. I'm going to be adding poly-fill, so this might not be a huge concern.

So, as of right now, I'm looking at 10's and 8's that require less than 5.25" mounting depth and .6 cf or below on enclosure airspace.

The 8" subs are perfect for size, but most of the frequency ranges are unacceptable, and even the ones that go down to 20 Hz or so might not be doing so efficiently. The 10" sub will fit nicely, there are several with the 0.6 cf airspace requirement, just the mounting depth comes into play.

Is there a certain distance that should be maintained between the magnet and the top of the box? If so, I would be pushed to an 8", with a depth of 4.5 or less...

Any ideas or recommendations? I'm open to any suggestions.