Note: the PCB holder doesn't have much grip. The 3D-printed screws are also a bit difficult to tighten/loosen.
The PCB holder already has some grooves to better hold the board. However, these weren't wide and deep enough to really hold on to the board so we've simply used a saw and widened them a bit.
The heater is a simple aquarium heater. By design the peak temperature is 37C controlled by a bimetal switch. This temperature is not high enough for sodium persulfate (Natriumpersulfat) etching.
The mod overrides the bimetal switch and puts the heating element into an always-on mode. A temperature sensor element was also fitted inside the heater just above the heating element to measure the approximate temperature of the heating element. There's a conversion table for sensor resistance to temperature.
Tests showed that controlling the temperature is not necessary, since the heating element peaks at around 55C and begins to slowly (read: several minutes) oscillate between around 45C and 55C. The modified heater was able to raise the temperature of the etch batch to around 47C which is a nominal increase of around 10 Kelvin over the unmodified off-the-shelf model.