Its incredibly easy to make a gas chamber. All you need is a styrofoam box, a small critter keeper and dry ice. The dry ice goes in the critter keeper which goes into the big styrofoam box that houses the rodents. You tip the critter keeper on its side, the dry ice which is already vaporizing floods the chamber with CO2 and the rodents die. Its really that simple. You can do dozens at once and freeze them for the next few months.
Here is a link to a bit more high tech version
http://www.ball-pythons.net/forums/showthread.php?t=55509
Here is a link to a bit more high tech version
http://www.ball-pythons.net/forums/showthread.php?t=55509