The smallest you can go would be better. Tight and humid spaces are the Ball Pythons favorite thing. If the rack is big, just use a separate heating element connected to the same thermostat and put the small tub on top of it. At least this way you wont have to worry about feeding issues.
6.5.15 Animals
1.2 Pomeranian's
0.1.15 Tropical Fish 2.2 Snakes:
0.1 2009 100% Het Piebald
0.1 Normal
1.0 Green Tree Python
1.0 Brazilian Rainbow Boa