The heater looks ugly but works a treat. Rather than the brute force treatment as before, I'm getting -10C and all clear of condensation on the glass. The maximum differential is not taking a big hit with the heater just warming the glass above dew point.
Given that I want this set up to be portable, and I have no wish to use a computer, and looking for ways to reduce the size of the power supply, Argon is not going to be all that convenient for me. Definitely the best way to go otherwise, but right now it's just a headache. I'll come back to it given time to think it all through.