Just a thought, but would it be possible to heat from the inside out?
If there was a sound, no, not this kind of sound like ultrasound, but the name for sticking a metal tube down your eurethra.
Imagine that either it is a heat pipe, or a tube with water that flows down and back, or a carefully temperature controlled wire with resistive heating.
By applying heat from within, and perhaps from the outside as well, it should lead to a more easily reached uniform temperature.
Heat pipes are amazing BTW, they conduct heat amazingly well, so that might be the easiest.
Watch this video:
The other end could be in a bath of water heated to the desired temp by a sous vide circulator (I have one, hmmm).
Of course, this means sticking a rigid metal tube in your penis, and that would need to sit in a bath of hot water below you, it’s doable but could be pretty awkward, also I suspect the heatpipe between the penis and the thermal reservoir of hot water should be insulated but might not be needed.
The electric option is interesting but it also doesn’t sound practical unless you include a thermometer in it and have feedback that controls the power supplied, can be done but a little tricky.
Water heated and circulated in is of course another option but I don’t see the advantage over the heat pipe, and it would mean a larger diameter to stuff down there, not ideal.
Maybe the heat pipe could be combines with an electric option, thermostatically controlled, I mean in theory a fancy temperature controlled soldiering iron if it can be set for low enough temps (unlikely) could be an option.
Here is a soldiering iron that goes as ow as 80C
Cxg Soldering Iron Adjustable Temperature Digital Display Welding Tools 110w Electric Solder High Quality Rework Tool - Electric Soldering Irons - AliExpress
It might be possible to hack a more simple dial based one, not really recommending this just a thought.