Hi Pete,
Here is the way I made a mast step. I had a set of saw hole cutters I think I bought at Walmart for about $6. Since my aluminum mast diameter is 1 1/2 inches O.D. I used the 1 3/4 inch cutter for my mast to drop inside. This leaves 1/16 inch clearance on each side of the mast after the cutter walls are subtracted. I ground the teeth off and mounted mine by fiber-glassing in place. When the mast is dropped into the step the large radius at the bottom centers the mast and keeps it stable with no slop or movement.

John