Good choice, Fledermaus) in my opinion, for field usage with changing lenses Helion is best, therm-app is kinda close) almost bought xq one, but found 2nd xp for similar price
The thread is m32*0,75, i didnt order adapters but was told it numerous times) my xp is focusing fine with 19mm ther-app lens, guess 6,8 would be fine too if someone sells them now. thermal expert seems to have shorter back length. Putting a lens designed for 320*17 on helion xq is kinda fine too, it has some icons on edges of pictures that i usually crop anyway. You can also try finding thermal-eye 3500as lens, its Ophir 65157 5,8mm f/1 in disguise(if you want to buy that ebay lot of 3 lenses, i'd get one or 2, you will need to allow shipping in Russia) I guess flir tau 7,5-9mm would work too, but they are a bit narrower. btw, i guess all lenses <10-11mm have big distortion, you should consider that too)
in the attachment is 3500as lens tried on therm-app without adapter, guess you know that usually ceiling with Armstrong lights looks more straight
But the circle almost fitted 384*17, though ophir claims it was for 320 only