I didnt have a heater for a while and really needed one (my room was like ICE)... In desperation I would occasionally replace (some, not too much) water with warmer water. The water wasnt *hot, and it was put evenly around the tank and was conditioned accordingly. I had a smallish 1-gal critter carrier in the beginning and could keep it under/by a warmth producing light with a barrier to its brightness and not its warmth.
Idk if itll help, but I wish you luck D: