Hobart type planetary mixers like the KitchenAid are not easy to cover. The only semi practical way I can think of is to purchase the hand guard attachment and cover the openings with foil. If you mean is there a way to cover the bowl while it's heating but not being mixed, just put the lid on it.
Though if you cover the bowl you don't get evaporation...which I want. I am aware there are a lot of variables, but as I have said before, once the process is worked out the results should be quite consistent. Except for ambient temperature and humidity (and atmospheric pressure) the mix formulation, bowl size, temperature, RPM, and mixing time are all controllable or fixed.
Edit: plastic wrap would probably be better and easier than foil.
