http://www.rodentracks.com/Rat_Racks.html

Large tub rack there. In the future if I build this I'll space the tubs out more so the lowest one is closer to the ground. As they are here, there isn't enough clearance to open the tubs with water bottles in. You could also use a watering system to avoid that problem.

4 tubs - 3 tubs with 2 females each. Rotate the male every 3 weeks. As the females are about to give birth put one in the empty tub and the other gets kept in the original tub. When the babies are about 2 weeks old you can put them all together in the original tub. If you need multiple sized feeders pull the males first at a smaller size and then 'grow out' the females in the tubs with their moms. They should be an OK size by the time the male gets back in the tub with them and would be gassed or fed before they get big enough to have litters or for the litter to be born if they happen to get pregnant.