Just an update: I bought a mini-mag Borofloat lens from Lighthound. I also added an o-ring around the lens so the lens stays centered. The o-ring size is 1" O.D., 7/8" I.D., and 1/16" thick. After I screwed the front part of the head down, it compressed the o-ring perfectly, and the sealing ability appears to be good. I am also using the stock flat rubber washer between the lens and the M60. The front part of the head does not screw down all the way, but it is not that noticeable, and the sealing ability of the stock o-ring in the head still also appears to be good. I'm happy with the result, and I am confident that my M60 is now well protected. I hope the info is helpful to you.