I was able to find a place that has a 3.5 inch OD, 9/16th wide, 3/8 inch ID axle. That would leave a little over 1/16 inch on each side, or more likely 1/8 plus the existing play.
I plan to use 5/16th halyards. That would seem to make it impossible for the halyard to get stuck next to the sheave. I think.
Would this work?