One thing to think about on the plastic BBs - plastic on plastic is pretty slick. I would expect that every time you twitch, the bean bag will shift as a few more beads move around. That might not be a problem in some cases, but I can think of times when it wouldn't be desirable, e.g. trying to get the exact same composition over several shots or a tricky shot where you're just trying to trigger the shutter without being able to otherwise support the camera. Off-the-wall suggestion, but try some lawn grass seed - it's fairly light, but the seed has "feathers" that will interlock when compressed and not slide around. Maybe some rolled oats/oat meal - just not the instant variety. Cheerios would work well, esp. if you shoot where there are hungry toddlers.