You could go with the dividers im supplying and hold it in place with a couple of suckers either side. Due to the divders having a number of 4mm slits up them ( not big enough for the bettas to get through) the heat will radiate around the tank
I have used dividers twice and both times my bettas squeezed through..dont ask me how..they didnt jump as it went to top of tank but they ended up having a free for all...so I dont use them any more!