The one I have is definitely 16mm

Sure I'll work on a V2, but the lens/filter the thumb comes with are actually something like 14.5mm in DIA. I could remake the model to fit the 16mm anyways but it may look a little funky.

Argh! Your model doesn't take 16mm glass unlike the actual runcam covers. Any chance of a 16mm version with a bit more depth to the glass inset?

Ok. I found some 16mm watch glasses on amazon. They fit perfectly in the original lens frames (my lens actually fell out and that's why I'm doing this) and probably will fit in your prints. I'm going to try sandwiching some ND filter gel behind the watch glass. https://smile.amazon.co.uk/gp/product/B09QXG2XWL

So I did manage to create a ND 4 filter by cutting through the 52mm lens. However, I'd suggest trying the ND gels on some acrylic. Using actual glass is difficult and will take several try's to do well. Spent my whole morning making a single lens...

Ah, ok... When looking around for DIY ND filters I saw people use some neutral density gel sheet. Unfortunately I suspect that it does not really protect the lens underneath. So I've also bought some 16mm diameter watch glasses to try.

BTW... "richardheath88" and "yith" are both me! I logged in with different accounts for some reason.

I'd try using some acrylic sheet and some ND gels. I bet that's a easier and cleaner way of doing it rather than trying to cut a glass one.