Just find a cool, level spot to perform the cal. I do mine in my basement. It's cool and stable. I used a carpenter's level to find a level spot on the floor. Let it set for several minuets to temp stabilize.
Something else I discovered that is worth remembering. When You do the IMU Calibration, the ambient temperature is important if done in a Warmish room. When You go to fly, drone seems to want to warm up to that temperature before flight. Do it in a cold room and it will be ready to fly immediately