Another way to get a near perfect fit would be to get it close with a sanding drum then get some stick on DA sanding discs and stick one onto the frame tubing and run the mount over the sanding disc stuck to the frame tube. This will get the fit near perfect with little effort. A solid billet mount would do a lot better here if you can get one. Then rough cut it almost to size then finish with the sanding disc stuck to the frame tube. Of course, the easiest way to do it would be to have it milled to the right diameter, but this can be done at home with a certain amount of effort.
I had to do this to a front mount in order to match the frame angle as well as shape and size so the engine would fit more secure in my bike. I used a die grinder and a 1" diameter carbide rotary file to do most the cutting involved with the angle change but once the fit was close it was time to stick a sanding disc on the frame and do the last part manually. End result was a LOT less vibration and a much more secure mount.