Unsure if this is of any use too anyone, but this is something a ABDL friend of mind showed me, and it helps me sleep on those hot London nights.
If your nappy is getting you hot and sweaty, consider getting one of those plastic gel ice packs, a long rectangle one is better than a fat square one..
Don't really try it with normal plastic ice packs because they are way too hard, the gel ones move about and are flexible.
Chuck the ice pack in the freezer, and once its nice and cold, just put it into the nappy.
You can put a booster pad, flannel, 2nd nappy or something over it so its not directly against your skin, since you don't really want frostbite, depends how cold your freezer is.
The cold slowly cools you down and you can just wash the ice pack off with some soap and water, since its waterproof.
If you are worried about it being in with food in the freezer, just seal it in a zip-lock bag first.
I can tell you that this has help me sleep though several hot nights.