Ive been using the pop up butler creek covers for a long time. I’ve only lost one, but have had a couple fall off that I noticed. Get em a bit small if possible. If they’re really hard to put on, they don’t seem to fall off that easily. I also broke one of the ocular covers with the push button to open. I haven’t tried the aluminum covers yet, but I imagine they’re great. But I’m cheap, and they’re not.....