Looking at your design I think it can be simplified for use with a standard mandrel. All you would need is say a short length of say 3/4" bar bored out to suit your mandrel with a countersink at the end to fit a standard live centre. Would that work? I have some 3/4" alloy bar if anyone wants to give it a try, I could knock one up? The only close size reamer I have though is 1/4".