I'm not sure if my mirror stems had some kind of plug on top that fell out at some point (guessing they did) but after doing some adjustment on the mirrors and needing to find a way to tighten one of the nuts these are now covering (tip: grind a socket down to a thinner wall that fits), I realized these probably shouldn't be exposed to the elements (rain). So I made these plugs that fit just right. I printed them out of TPU and the fit is just perfect. I didn't try other filaments, but they might work too. I wouldn't use PLA for sure since it'll melt on hot days. TPU is recommended, but PETG or other UV resistant filaments might work.