Bob N1KW found that by simply increasing C1 to 2.2 uF (just parallel the existing 0.1 uF cap) the keying shape is greatly improved with about 3 ms attack and 6 ms decay. That is much better than the key click machine it was originally with about 0.5 ms attack and decay times.
Rob AG5OV picked up a cheap temp fan relay from Amazon.
These are also available on Aliexpress for less than US$2.00.
Rob superglued the probe to one of the heatsinks on the finals and then played with ON temp and hysteresis temp. He settled on 50deg for turning ON the fan and 35deg for turning it OFF. Rob says it works great.
Kees K5BCQ is no longer selling his AGC and Audio Pop fix boards. It was a good run with 888 sales and many happy customers!
However, Nik VK4PLN who already supplies a number of small add-on boards for the µBITx intends to release a slightly different version with components face up and Through Hole along with an area to install a pot to govern the release time.