10 mm thick spacer fits 4 inch electrical box.

I didn't take into account that the conduit couplers would interfere with the GFCI outlets. I needed less than half an inch extra depth for the outlets to fit. There was no way I was tearing apart enough work to replace the box with a deeper one. And after buying the wrong box extender (twice!), I gave up and designed this spacer.

I printed it in PETG, which should be good to 150 degrees Fahrenheit (a lot hotter than what my garage will get to).

I used three perimeters for a little extra strength.