Petsmart actually carries bales bigger than 2.5 cu ft. They sell 4 cu ft ones for 12.99. Also NEVER use rabbit food for rats as the alfalfa and other ingredients aren't good for them and you might just have issues down the road.

If yours is the critter cage with the lid that slides off from the side, then you have 4 grooves in the plastic to fit wires and other probes through, just cut the metal mesh to size and it will close and snap shut. You can fit a water bottle in the other larger opening too.