Here is one source for spawning cages, also called "live cars" or "holding pens"
http://gmocart.gmotion.com/store/mer...dingboxes_nets
I have the 4 ft x 6 ft, but wish it were larger. A PVC pipe rig is used for buoyancy and to keep the top of the cage above the water level.
http://www.raingarden.us/spawningcage.jpg
I think Maurice said he uses these as well.
-steve hopkins