I just bought some stainless steel washers with a 5/16" hole in them. Mounted 3 of them on a long 5/16" diameter carriage bolt between two nuts and spun them down on against a grinder by hand. A file with the bolt in a 3/8" drill may do the same, but I couldn't wait that long.
When spinning the washer's O.D. against the grinder, and once they just began to grind off the two nut's hex edges, they were about the right O.D. for the assembly. It was really easier than trying to drill out a smaller one for the proper I.D. Mine took 3 (one outer + stock washer, and 2 inners).