Ridgid Oscillating Sander Large Washer 1 3/4"
This is a printed version of Ridgid part TTI805553-30 WASHER 11/32X1 3/4X1/8
It's a fairly trivial thing, but I managed to misplace the original metal washer and they are out of stock on the Ridgid parts site. I printed it with petg using a 40% gyroid infill for sturdiness and it works just fine.