We use this....http://www.arbreptiles.com/cages/rat_rack/ratrack.shtml

It works out really well. We move our rats around quite a bit, but when breeding we usually only do 1.2, although I think there is space for 1.4....maybe 1.5, just for breeding. If you use more than 2-3 females, you should probably separate them for birthing. Ours have not had any issues with communal nesting, though other people have, and to avoid the situation entirely, separate birthing is always the safe bet.

This summer I am hoping to build a rack like this...http://www.arbreptiles.com/cages/rat...ing_rack.shtml

That way, we can increase production if we want, and I think I am going to make a level or two with 1/4" hardware cloth for our ASF's. Also, another benefit with the separate birthing is that you can know for sure which babies came from which moms. This helps a lot if you are selectively breeding for markings, colors, litter sizes, or whatever else your heart desires.

Hope this helps.