A taste of the UK Field Target Scene - BFTA 2017 Champs

Sure...Its made by BISLEY if not mistaken...in the UK, they call it a "pig's ear" in colloquial parlance...

>>> Google "Bisley rear scope eye viewer" <<< to be used for whatever Brand of scope you own ; different types & styles (NB: the Best is called "DG scope enhancer")

Yup, like a floppy pig's ear...totally floppy and enveloppes the eye ...

So the thing is, you are supposed to drop the rubber eye piece into some boiling water...remove it...let it cool a little (but not too much) then quickly start sliding it onto the rear ocular part of your scope...sure, you can try it without softening the rubber up...and if works great ! otherwise, as said, soften the rubber by dropping it in boiling water...remove, wipe off water drops, and it then stretches more easily over the 30mm rear objective part...

here as example; Bisley: https://www.amazon.co.uk/Savage-Island-Scope-Maximiser-Bisley/dp/B0047U75R0

DG scope enhancer: 

https://youtu.be/XhV8UCh9Lbg