For small tanks, you can use a small 30 to 60w lamp to help it stay a little above room temperature. If you can, get a 25w submersible heater, it can heat anything from a 2.5 to a 5 gallon tank.
Cold weather plays havoc on a betta's (most tropicals) immune system. They tend to get fungus and that can kill them overnight. Anything below 72F is pushing the lower limit.
No mircrowaves. Blankets will help. Check your local bargain finder for a 5 to 10 gallon tank. Most sell used at $15.00 with heaters and lots of extras.