Practice guitar for commuters

ShredNeck?
LOL, I remember going on their forums and just seeing a bunch of threads linking to porn way back when the ShredNeck came out, haha.

Honestly, there is probably no reasonable way to practice guitar without an actual guitar IMO that's reasonable and cost efficient, as far as I know. You'd probably be better off performing finger stretches or using one of those balls you squeeze over and over again, just to get the blood flowing.