I originally tried to stick them both in my pretty large case from BuildRedux, but it would overheat and shut off, so I just bought a mining "case" - basically flat steel bars rivetted together. It can supposedly hold 8 GPUs, but I only have my 2 3090s on it and even then, when they are both working on something, it kicks off a LOT of heat, but with a fan blowing on it, the GPUs never reach critical temperatures.

Last winter, I had it in stored in my AC return closet, and used it for crypto heating, and my AC/Heat was just set to circulate. Kept our apartment at around 70F even when it was in the 30s and 40s outside.