I wouldn't do it out of plastic as it will probably deform when clamping up. I have a drill chuck arbor I need to do the same thing to. I was thinking of just putting some stock in the 3 jaw, trueing it up and clamping the drill chuck to it.
I also have a 3-2MT adapter which I was thinking of turning the outside parallel for just that purpose as well. According to the marks on the outside I don't think it is hardened but haven't tried filing it yet. Then splitting it along the length so I could clamp it in the 3 / 4 jaw and do the same, or maybe turn the outside parralel, loctite the 2mt arbor, in drill, tap etc, stick in oven with tea and then knock the altered arbor out when hot.
I also have a 3-2MT adapter which I was thinking of turning the outside parallel for just that purpose as well. According to the marks on the outside I don't think it is hardened but haven't tried filing it yet. Then splitting it along the length so I could clamp it in the 3 / 4 jaw and do the same, or maybe turn the outside parralel, loctite the 2mt arbor, in drill, tap etc, stick in oven with tea and then knock the altered arbor out when hot.