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.

You need to tap the center holes of the extrusions to 5mm. The hole is already the correct size for that.

The corner blocks require low profile head screws. I used button head screws.

Spool holder (I'll public a STEP file soon)

The little clips are used as wedges install the acrylic panels,

The spool rolls on 1/2 EMT electrical conduit with 5x15x5 bearings pressed in. The handle is made from the same material.

more details here...http://www.buildlog.net/forum/viewtopic.php?f=16&t=1625&start=40#p15958