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.

I don't think there is an issue with how close it comes to the back as long as it doesn't actually touch the back so not to create a rattle and block the venting for the voice coil depends on the sub. As for a suggestion for a 10" sub in a .5 cubic foot box look at Boston Acoustics. I haven't installed one of their subs yet but, if they are anywhere near as good as their component speakers they are well worth the money. The dimensions should give you some extra breathing room as well with the 4.75" mounting depth.

kicker solar barics are nice in small enclosers.but they need more power than 200 w.

I certainly love the pair of 10's I have in My Eclipse. I got the BA's partly on how great their Component set I have in the front is,a nd partly on that very same airspace requirement (I put mine into an enclosure sunk into the spare tire well).

There was a product called "Variovent" a few years ago, I used them and believe I had great success. I say believe, because it was about 10 years ago and I didn't have such a critical ear then. That, or some other form of "aperiodic membrane" may allow you to perhaps have a pair of 8" or some other configuration to get what you are looking for. Thinking quickly about some Bose speakers, or headphones for example, I wouldn't necessarily conclude that an 8" can't make good bass. Of course, it depends on what you're loking for and what music you listen to.

I'd check the manufacturers websites. Almost every company seems to have speaker dimensions and box sizes both on their websites ready for download. You may come across something that fits your requirements. Good luck though, the trend these days seems to be towards giant magnets.

I looked into the Bostons, looks like a killer sub.

Yet after all the trouble, I redesigned the box in CAD format, and it looks like I'll be able to get 1/4" clearance on my current Kappa Perfect 10 (5-3/4" mount. depth). I'll either be removing foam from the underside of the upper fold down compartment, or just compressing it enough to get a level armrest/table.

Thanks for the suggestions.

Cool! As long as it's built solidly, you really won't have an issue. The Perfect 10 is a great sub as well. Have fun!