I bought some aluminum flat bar 1"x3/8" by about 24" long, it cost about $8. If you don't want as deep a clamp you could certainly go smaller than 1". 3/8" is the critical dimension since it is approximately 1/2 of the 3/4" washer diameter. You don't want to go much thicker than 1/2 the diameter of the washers since you just end up sanding it off and that gets tedious.
Cut two pieces of bar approximately 1-1/2" long, mark a spot centered in the top piece and drill a 7/32" hole (1/4" tap size) through both keeping them clamped down and aligned. Too make it easier to bolt together I tapped the bottom of the clamp for a 1/4" screw. The top was drilled out to 1/4" and then counter sunk enough so the head of the bolt I used was below the surface of the top, I also cut the screw down such that it didn't protrude out the bottom. Dont' forget to deburr all holes so things clamp together tightly.
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