Thanks that's is exactly what I was looking for, I knew I'd seen one somewhere.
There's one on Thingiverse for 3d printing:
https://www.thingiverse.com/thing:4574121
Happy to print you one if it is what you need?
i just do the same as angrybear
I printed a couple off:
https://imgur.com/a/Fvff21s
And here on my Hatsan pump which has a tube diameter of 31.6mm
https://imgur.com/a/KNxClTc