I have old eyes too, and wear safety lens bifocals all the time including when shooting. For me, the Gene Davis boosters work just fine. I have 4. I use them on 4 Weaver scopes--the screw on 3X ones. and on Mueller scopes, the 4X slip on ones. I had an interesting experience with scope boosters. BTW, 3x does not mean that your view will be 3 times larger, it means that you have added 3 power to your scope and etc.... One Mueller scope refuses to adjust so that I can get a perfect reticle, but all my Weavers are nice and sharp, and my other Mueller, too.
Two of the lenses I bought from Gene's website, bullzeyepro.com or something like that I bought 1 3x and 1 4x. The other two 3x screw ons I bought from an acquaintance at a club to which I belong. He was fiddling around with them at a bench next to me. I could see he was screwing the eyepiece in and out at random and looking through the scope and cussing. I talked to him about them, and he vented his exasperation in such a way as to convince me that he would never be happy with them. I offered him a look through mine which I was using that day and he said he thought that one worked, but his did not. He was going to either send them back or throw them downrange. I tried one of his on my Weaver T36. I offered to buy them and he said he'd sell them for 50.00 each and I took him up on the offer.
The only difference between his usage and mine was that I followed the directions EXACTLY and he did not. I even pointed this out, but this guy was not in a mood to hear about what he had done wrong. This worked out well for me--usually I have no money when opportunity knocks.
This is not to say that these lenses will surely work for everyone, but I'm willing to bet that quite few that have been returned, were, for the same reasons that guy would have returned his.