Necessity is indeed the mama of invention.
Wow, living less than a five day drive, and an international border away from Stew Mac.LUXURY!! Aaanyway. Let's see. You can stick the pickup onto the back of the bridge facing the tailpiece.
You can also stick it to the underside of the skin right underneath the bridge feet.
These little guys are terrifyingly sensitive, so the foam is a help in toning them down a little. That's not to say that there aren't other mounting mediums that wont' work. Go nut, I says.
This is neither a high tech, nor a high fidelity fix. But, if you can't afford anything else, and you gotta be loud, this is certainly one way of bringing home the tofu. Also you can control feedback better than with a microphone in close proximity to the vibrating skin.
Incidentally, it's easier to mic a banjo if you use a resonator. Some folks do, and some don't.
There. I'm done. Again. For now.
Cheers and banananas.