Knock out miter-slot slippage

My tablesaw has T-shaped miter slots, so when building tablesaw jigs guided by a runner, a washer on the miter bar prevents the jig from lifting out of the slot.

The problem is locating washers the right diameter to fit the slots. While doing some electrical work using metal outlet boxes, I discovered that the ³⁄₄ " knockouts fit the miter slots. I located the center and drilled a countersunk mounting hole to fasten it to the runner.
—Michael Tenore, Bronx, New York