Honestly I think 1gallon is too dangerous to heat because it could easily overheat. I have nothing against 1gallon if you keep your room temp. warm enough, but I really do think a heater for a 1gal is not a good idea. I keep my bettas in clear storage containers. I keep my bettas in 3 and 5 gallon clear storage containers. The 3gals are really 4gal but ofcourse I don't fill it up all the way. The 3gals that I use are like this, and I drilled large holes on their lids:
They cost 2.99 at big lots. They have more space width wise than depth wise which is why I think they are a pretty good option, they are even wider than my 5gallon ones.
Btw the 5gals cost me about 6.49.
I think 3-5 gallons is pretty safe to heat. Add some gravel, plants, and decorations and they are a great CHEAP homes for bettas.