Buy Merit Optical brand
I have a Merit diaphram thingy for pistol shooting. Old eyes see a clear rear sight notch, front sight post, and sharply focused round bullseye -- at the same time!! The best $65 I ever spent 10 (?) years ago. A buddy has an "off brand" because the Merit was too expensive. It doesn't work as well as my Merit. We traded eyepieces at the range one time. He was amazed at what he could see clearly and focused. I saw my target as a black, fuzzy blob -- the group proved it too.
The rubber suction cup will not scratch a plastic lens. It does not work real well on lineless bifocals unless you find the 'sweet spot' in the lens. I shot a handgun silouette match, and every old guy was wearing a Merit.