Mine is a Radio Shack 8" 'full range' speaker that I got for $8 when they discontinued their speaker parts. I put the leads of the speaker through half the windings of a transformer from a DOD passive direct box (very cheap), in reverse for step-up.
I don't know what the ratio is of this transformer, as I can find no info on it, however I'm guessing it's at least an 8:1 or maybe 10 or 12:1. There was a center tap, so I just used half the windings.
It sounds phenomenal layered with a second, internal mic.
To mount it just used the box it came in.... cut a hole and used screws to mount the speaker. I punched another hole and put an XLR connector on it, so I can plug it in just like a regular old mic.
I don't know if the transformer was necessary, but I had it already, and it sounded better this way, so I used it.
I actually think one of the reasons it sounds better through the transformer is probably because it is so cheap. I'm sure there is all kinds of phasing going on in the low frequencies, which would make them a bit more noticable without the need for eq (that is only my assumption based on the idea of the BBE sonic maximizers, which alter LF phasing to make them more pronounced without having to increase level). That is my guess anyway.
In any event, it's cheap and sounds great. I love to watch the faces of clients when I show them the difference of the kick sound with and without it. By itself the thing sounds like absolute poop, but when you remove it from the overall mix, the weight and solidity of the kick just disappears. If you haven't built one of these, you really need to. WELL worth the effort!
JC