I havent found any aquarium heaters that go up to 38c-39c, as people dont normally keep fish at those temperatures.
Think the ones I saw go up to 33.8c. Perhaps you could test how long it takes to develop properly at this temp with your chems and if the results are any good with E-6 or not? I think contrast might be lower even with corrected development time is my feeling.
I'm not sure if you put an aquarium heater designed for a much larger volume of water in a small bath lower than what its designed for it'd both heat up over it's max normal temp and also keep that stable.
I've used the tetenal kit C-41 @ 31c with the instructions provided and results were very good (I've tried flexicolor @ 20c and 24c and results are crap, thin density range), so perhaps a tetenal E-6 kit would provide instructions for 31c.
For temperature regulation what I do is use a plastic tank, and pre-soak my film in hot water above the temperature required for processing to heat the tank up for a few minutes before placing my developer in at the right temp, plastic is a good insulator, where as steel is a great conductor and the worst insulator and will change temps quicker on you.
Pre-soak + surrounding water bath in a plastic tank should get you a very stable temp if you're really paranoid. If you have a heater regulating a bath temperature, steel may be the choice of tank
edit: 30c sorry it gives.