My eyepiece diameter is 42.5mm but size 42/44 does not fit. I placed a thin film of Lexel around the inner perimeter to hold it in place. The retaining ring does not stretch and does not provide a secure fit for my monocular. Also, the strip between cap and ring is too short to fit through my adjustable eye relief. So I cut off the strip on the cap and made a longer halter out of 60lb braided line. Those complaints aside, it feels solid and well made. It's almost a rubbery silicone material that should hold up well to wear and tear.
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