I'm going to agree with going with a drilled and tapped rail. I have them on two of my Mossbergs. My most recent dot was a green dot by Holosun. It has a 30 MOA circle dot as an option. I mainly got it to use at night with my red spot lights, but now it's become my primary shotgun.
As far as sighting in, I use wrapping paper. It sits at the perfect height to get it zeroed. I start out at 40-45 yards, get my pattern as centered as I can, then I move closer and note what the pattern does as I get closer. I log that away in the old DOPE chart in the head and out I go.