just wondering if anyone uses a heat box to warm up the epoxy before using it ?

Heating epoxy is a waste of time and energy if your shop, boat and fiberglass is not the same temperature.

The moment you apply a coating it will be the temperature of the wood or fiberglass it touches along with room temp.

If you don't control the temperature of the epoxy/hardeners while heating them how high a temperature do you allow them to get?

If the epoxy/hardeners temperature goes over 90˚ F. you're going to cause the hardener to kick the moment you mix it in the cup and then when you apply a thin coating it will chill to room temperature. This is not useful.

If you're epoxy is old and cold and crystallized you can use heat to bring it back to normal viscosity.

If you want to wet-out fiberglass heat your shop to 80˚ or wait for a warm day and the resin will stay thin and wet-out fiberglass transparently.

Epoxy resins will become much thinner at 82˚ than at 72.