This is the gas chamber I made today.

Went to a military surplus store and bought a cannister intended for paintball co2. Had'em top it off,
bought a hose and fitting, drilled a hole in a cooler, screwed in fitting, dropped in live rat, closed the lid,
opened the valve and it was done. The rat was put down humanely. I only put in one for the test, but the cooler can hold several rats at a time. In fact, I'd bet that the more rats I put in, the less gas it would take to put them down.

All the equipment cost less than a hundred bucks. The canister, although small, holds quite a bit and the
guy who sold it to me said he estimated that expanded, the gas would equal about 600 cubic feet. That's
plenty of gas for my small time operation. And will only cost about 4 bucks to refill it.

I do have to hold the lid down tightly to keep the gas and pressure in. But that's not a big deal. Just takes a moment.

What do you think?