I needed a way to mount a cable clamp for the wire loom on my XY axis but didn't have any metal corner blocks. I just made one side flat leaving the remaining M3 hole in place. Same thing as grinding the nubs off a metal corner block.
The hole for the cable clamp was tight enough that I didn't need to add a nut. See other photos for my installation.
I used 30% infill to get it a bit more strength. Part doesn't need to be super strong in this application but there will be subjected to some twisting motion. 30% would probably be good enough for most structural applications.