None
I was considering using a defog fan, since I plan to fly in winter. After some thought, I did not want the permanent magnets in the fan to affect my compass reading, and decided against it.
Now that I am flying the plane, I find that I have not needed it yet (25 hours). A couple of times the windscreen fogged up as I entered and closed the canopy (slider). Once engine started, it did not take long to clear up (a minute or two), and I could taxi safely.
I do have dual mufflers with heat muffed plugged in series, which provides loads of heat. Temperatures flown were from 0 F to 20 F.