Ugh, I can't do the whacking method either - aside from the squick factor, my aim is terrible.

You don't have to go all fancy with a CO2 chamber. A plastic bin (like a storage tote), a small kritter keeper, and a quick trip to your local grocery store for a cheap hunk of dry ice is all you need. Dry ice in the kritter keeper, keeper in the tub, rats in the tub. Close the lid and give it like... 15-20 minutes.