Here is an alternative Co2 chamber.
You need a 5 gallon bucket (with Lid), a small "tupperware" type bowl (with lid) that will fit into the bucket, and a source for small chunks of dry ice.
Take the tupperware type bowl and drill a bunch of holes in the lid (most of mine are 1/4" holes). The "tupperware" bowl and lid is to prevent prey from coming into direct contact with dry ice. Contact with the dry ice will cause frostbite and limbs to fall off (on prey).
Get about 1lb of dry ice. I smash it with a hammer to create more surface area, then pour it into the "tupperware" bowl. Then add water til it is at 2-3" above the level of the dry ice (once the water hits the dry ice it will immediately start to off gas, so you may want to pre measure an amount). Put the drilled lid back on the "tupperware" bowl.
Put the "tupperware" bowl into the 5 gallon bucket
Place the bucket lid on LOOSELY (do NOT make it air tight or the lid will pop off and can cause injury).
Start putting in the prey you want to euthanize. I put 2-3 medium to large rats in every 30 seconds. They will pass out in about 10 seconds and expire within 2 minutes (usually less but 2 minutes is for just in case).
This method will produce Co2 for about 7 minutes. More water can be added or the bowl shaken to produce more.
This can easily be scaled up for more at a time ... however do not use Iris tubs (these crack real easily with drastic temp changes), or glass fish tanks.
If anything needs more clarification please post back.
Bryan