SMD reflow oven baseplate material
You could use sand. I have seen people use a frying pan filled with sand to for this. See this: https://hackaday.com/2019/06/05/solder-smds-with-a-pan-o-sand/

It will spread the heat evenly and touch the board evenly as well no matter what shape it is. You should be able to get very fine sand in hardware stores. It's used for mixing concrete for example.