I made these corner blocks for Misumi 20mm x 20mm extrusion. Misumi sells similar blocks but they are close to $6 each and I need a lot of them. I also designed it to have one screw compress the layers to make them a little stronger.

This type of corner block allows you to use the slots for acrylic panels.

There are three versions. A standard corner and two corners (left and right hand) that act as feet.

I used them to make a nearly air tight spool holder. The extra parts for that are here too.