when I was rescuing bettas I used the critter keepers and heated one and then stacked another on top. I found the water in the top one was quite enough to keep them comfortable. Be everso careful if anyone does this as I managed to cross infect a couple of fish. Havent got many bettas at the moment got lots of critter keepers though!
you know you can stack critter keepers, one on top of the other. Put a heater in the bottom tank and the top one keep warm as well. I never filter my bettas tanks...got real weed in and they are lovely and clean