Sorry to rain on your parade, but I've been doing these for several years now....as you said, cut the shotgun cases down with a Dremel, pop out the primer and its surround then re-fit using an appropriate size eyelet then super glue a used large rifle primer back in the center.
With the rifle ones, I cut the base at about 3/8" then put six cuts in the body to flare out once its fitted to the leather..I hope this makes sense, the pics may clarify...
Roger