How are your fish doing? I thought about it and you might be able to get away short term with a 2.5 gallon tank, three tetra, a snail and the betta- just change the water frequently- test it for amonia and change it when the amonia is high. You can get a 2.5 gallon tank pretty cheap and it can be a QT tank later. When I rescued my betta I put him in a gallon-sized bowl for the first two days and he did okay and got healthier- but I had trouble heating it.
Anyway, I hope your fish are doing well and your snail has stopped being an escape artist.