Just curious, what model BF do you have with only one bottle mount? My old NWT had three, I'm pretty sure, the Crusoe has two, Pocket Tourist has one.
I've used both the older and newer versions of the Twofish cage, usually mounted on the handlepost, toward me, on my Dahon. Yes, they do slip to the side sometimes no matter how hard you cinch them, but it's not really bad, I just push them back from time to time while riding. They're not flopping all over.
I suppose one alternative might be the Minoura bottle cage mount. (
http://bikepartsusa.com/product_info.phtml?p=01-170957 ) You put it on a frame tube, and mount a regular cage to it. I put one of these on the head tube of my Twenty. Perhaps you could put one on the back of the seat post or even seat tube, though it would be hard to reach while riding, okay for a spare bottle. It could probably go on the stem, too, but that might impair the quick-fold. The metal strap holds it very tight, and doesn't seem to have scratched the paint on my Twenty, but you might want to use a rubber shim if you did go this way.