A technique I spotted in the studio world, but works okay live if you've got some EQ available:
Get a '57 (or whatever instrument mic you like), put it back in the pouch (still cabled), zip it up and wedge it under the tailpiece of the bass, where the strings attach.
While mics generally aren't designed to be contact pickups, this has worked quite well for me in situations where a stand-mounted mic simply isn't applicable.
The sound is a little bottom-heavy, with plenty of the thump-thump from the plucking fingers coming through. Easy enough to attenuate that, but most bass players like it.
Chris