Monkman

have a look on the real steel pages on facebook or have a look at mapleleaftactical (ambiguousdave on ebay) on IG failing that tactical optician can make you sacrificial lenses.

https://www.ebay.co.uk/itm/PVS-LIF-Light-Interference-Filter-Sacrificial-Lens-Night-Vision-Norotos-Wilcox14/273461713010

Polycarbonate sheets are cheap, easy to cut with a jigsaw (or a hacksaw with patience), and 2mm should be enough to see off most BBs. All of my sights are running DIY'd polycarb protectors, and my torches now have polycarb "lenses".  It doesn't have to be neat: one sight just has a square slab of polycarb with holes in each corner and thin cable-ties lashing it to the optic.  Looks a bit trailer park, but it works fine.

the one i have the previous owner was kind enough to have left in a homebrew polycarb lense as rogerborg suggests, seems like it'll do the job and remember it's a night vision sight, to be used at night, so looks arent important