throwing another into the mix, I will probably get laughed at for this but SMK do a rear target sight which is available for £40
https://southlondonairguncentre.co.u...diopter-sight/
I have one I bought a while back, I am tempted to put it onto my HW80 with the CS600 Tyrolean stock.
I suspect it is for their QB78 target CO2 rifle so it would probably need drilling and tapping for a recoil pin (e.g. a short M4 or M3 screw, black).
It is adjustable for windage and elevation.

Sorry I got a little confused and posted this on an older thread, but I have just copied it across.