red crab potentially at large

Greetings.  I have a red crab.  I know nothing more about it other than, its a small red crab.  When I bought it, the lfs said it was a red crab.  I said, "cool" and took it home.  He looks kinda like an emerald crab, but really red and no hairy legs.  Anywho, its about an inch across, legs and all, and likes to climb to the tippy top of my rocks and take a breather.  Its in my 12g nanocube.  I'm afraid it will wander into the filter in back and I'll never see it again.  Anyone here have similar problems with nanocubes and any solutions?  Maybe some kind-of rubber lining I can somehow attach to the back so it doesn't climb across into the back?  Windshield wiper blade?  I need something that will grip to a ~0.2 inch wide line of plastic . . .

OOh, I hadn't thought of that!  Great idea.  Maybe I can get some glass cut at HD and somehow mount it on there . . maybe with some silicon . . . thanks!