I had a machine shop make an adapter for my Losmandy mount to a pier in my observatory. This adapter worked great (until I bought a new NJP mount), and fits a standard 4 inch pipe. The Losmandy mount transition piece (that the legs normally go into) fits precisely over the top. I drilled and tapped 3 set screws into the bottom of the transition piece to hold the transition piece in place on this adapter. The inside diameter of the adapter is 3.975 inches. It fit very snug on my 4 inch pipe. The picture below should help in the description.
The piece cost me $225 to have made and I am selling it for $150 shipped.