Weeell, from that point on, I personally would hesitate to advise you.
2G is considered bare minimum for heaters, for a reason.
I think bettas above 75F in water are far, far happier, than bettas between 70 and 75F, personally. My female, who is in 80 degree water, is extremely active and never stops moving. She's like a little trout.
A 2G bowl doesn't take up too much room, and a 2G All-Glass tank doesn't take up too much room, what harm is there in investing in one of those, so you can get a proper heater?