Historically, I have been using 5mm or 6mm PMMA sheets to make lenses for the goggles. One of the most challenging part of making these lenses is grooving the side so that the assembly clip can slip in place easily: position from the side of the lens, depth and, width are all important to ensure a water-tight fit for the lens on the skirt. So, historically, my DIY groover worked well for PMMA sheets but the resulting lenses were easy to scratch. Picture water, sand, waves, etc.

Eventually, I ordered a set of different types of Polycarbonate sheets to make scratch-resistant lenses because, PC, is a lot tougher than PMMA...

Sadly, what makes PC so much better from the point of view of being scratch-resistant, makes it so much less fun to groove with my DIY groover: the distance, and depth parameters keep going off. It takes at least 10 to 15 minutes to groove a single lens. I have no doubt I could learn to groove PC faster and more accurately but, really, now is the time to order a GM-150 groover from aliexpress. Hopefully, making new lenses will become less painful.