This is my replacement for the round cup washer on the mini mill.  It is a piece 5/8" x 4" x 4" piece of sheet steel.  The important bit is that I relieved the side of the plate against the column about 1/16" for clearance to put the force of the pate against the sides of the column.   I drilled a 1/8" hole in the plate and column for a pin to keep the plate from moving while being tightened.