Years ago I noticed that if you tap on the bottom of a glass of banana smoothie and listen to the top, the doongy sound is lower than you'd expect and the pitch goes up as the bubbliness decreases. It's a similar effect when you thump your chest and the pitch of the thud is rather deep: Your lungs are a lattice of bubbles (the alveoli). And when you stir a frothy drink like a cappuccino, the pitch goes up as the bubbles decrease. No one's been able to explain the physics of it to me satisfactorily, but all I know is, bubble make things sound deep. I used this knowledge to make a compact stompbox (a little fake kick drum). I've tweeked the design a little over time, but basically this is how it's done:
Find an abandoned leather CD wallet (or buy a new one if you have to... I haven't tried vinyl), take out the sleeves and fill it with 5 layers of bubblewrap. Use gaffer tape to form a thin L shape gap around the edge leading to the mic hole. Then gaffer tape the zip side of the wallet tightly together so it's a wedge shape. And that's it!
Now for the mic: You can just poke a SM57 or a similar non pop-sheilded mic with good low range in the side. Screwing the pop filter off a SM58 (or the like) works too. What I did was use the capsule of a Behringer XM8500 mic (about $40 on eBay). It's tragic to waste all that metal and pop shield, but you can't buy the capsules on their own (yes, I've tried). Unscrew the pop sheild and yank out the capsule, then use long nosed plier to break of the wires as deep into the handle as possible. Then solder it onto a lead with a male XLR at the other end and bob's your uncle.
The air gap around the edge seemed logical to me, to produce a little boof of air, much like a beatbox kick sound. I did try it without the bubblewrap and it sounds awful... it's all very curious. But hey... it works!