A couple of ways. I added a thermometer fitting by using a Greenlee punch to make a hole, then I added an adapter with 2 silicone washers, one on each side to mount the thermometer thru, I worked fine.
On another project I used a larger Greenlee punch to make a hole thru the sidewall of my evaporator and I used a fitting that mounted thru the hole, giving me an opening to use a threaded adapter in the opening. Off hand I forget what that adapter was called but someone will chime in with a name. It has a gasket on each side , and a backwards thread and nut on the back side to tighten it.
Dave Klish, I recently bought a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.