A 20 gallon is pretty big for a baby Ball. You might get away with it by sticking a lot of crumpled newspaper in there, but I think it would just be easier to get a 15 qt. bin and a foot of 3" or 4" Flexwatt. You don't necessarily need a rack for one bin... just secure the lid with binder clips or get a bin with a locking lid. That should last you a few months, then move up to a 32 qt. or you should be able to use the 20 gallon by then.

Also, don't forget a decent thermostat. That's probaly the best thing you can get for your snake.