I modeled an adapter to use a 12mm macro lens.
For this small diameter to be usable, the lens must be fairly close to the camera window, otherwise the entire surface of the sensor is no longer usable.
I was inspired by an existing design whose dimensions were too bad to fit properly.
It works very well with perfect and repeatable centering of the lens.
Advantage compared to the use of 20mm lens:
cheaper lens and reduced print time because the parts are small.
The camera operating in visible light is masked but as the parallax is very bad in near vision it does not matter!
New: test with an macro lens of 38,1mm lens focal (see picture)
The 12mm diameter does not cause any annoying distortion on the Uti260b, this is surely not true on all thermal cameras!
Here are two thermal images that show it, it is a printed circuit board with 2.54mm pitch pads
The first with a 12mm lens with a 50mm focal length and the second still in 12mm with a 38mm focal length.
The distortions come mainly from a bad paralax to avoid reflections (mirror effects)
It can only be better with 20mm lenses but it is very acceptable with 12mm!
See:
https://www.thingiverse.com/thing:5365242/files