I feel a paludarium setup would be a good fit for American bullfrogs. The construction of the enclosure will depend on how much time, money and effort you are willing to put into the construction and maintenance. You could make a full-scale planted living paludarium(higher cost and work) or a paludarium with fake plants. I feel a land-water split of 50-50 or 40-60 would be comfortable for them. You could make a naturalistic backing or just cover the back with an aquarium backing sticker(there's probably a proper name for those). As for size of the enclosure, depending on the size they donate with him/her, you may need to upgrade. American Bullfrogs are amazing jumpers, and need a lot of space. I'd say a minimum 40-55 gallons for frogs like these. Not only are they jumpers, but they tend to WIG OUT when you come near the tank. This usually means a full power jump in an attempt to escape. In too small a tank, they'll smash their face first into the sides a lot. Adult frogs can be fed every 2-3 days. Crickets, roaches and nightcrawler worms are good foods. Silkworms and Hornworms are also good. If you have any follow up questions please don't heaitate to ask! I hope this helps.