Update 4 years later and being more experienced; do not use this. PLA is not stiff enough for a printer frame. Use aluminium or steel brackets. Blind corner joints + 2028 brackets create a very stiff corner at just 50 cents worth of hardware. Use 2028 brackets instead of 2020, it makes a big difference.

I wanted a stronger corner bracket than the simple corner pieces we all know.

The holes are all 5.2 mm, which results in 5 mm holes on my printer. The key-nubbins are 6 mm wide. If your extrusions have slits smaller than 6 mm it would be simple to cut off a little material to make it fit your particular extrusions.

*edit 3/7/18 added extra nubbins to more easily allign the two extrusions perpendicular and added more clearance in the corner.

Please share if you end up making one of these yourself!