What Greydog describes is essentially the same as my method, but a bit simplified. I don't like the idea of bedding the scope directly into the epoxy. I worry about having it stick and needing to knock it a bit to get it free, so I use a length of precision ground 1" steel rod to form the epoxy bed.
Even that may not be a perfect fit. As Mule Deer noted elsewhere, scope diameters are not always true to spec. Folks, when you consider the various components made by different manufacturers, it's taking a lot on faith to expect everything to fit perfectly. There's this thing called stacking tolerances. Sometimes you get lucky, sometimes you don't.
Paul
