I found this 20x20 corner piece but it didn't fit my needs. I used the idea and made my own. This has m4 recessed holes and a 28.25x28.25x28.25 block in the center to make it take the place of the Core A8 corner pieces when used by 20x20 extrusion. In the box there are holes in the center if your 20x20 extruded aluminum is tapped. I printed these in Petg and they are a lot more ridged then the original corners.

You can use whatever settings for infill. I suggest a 2.4mm wall so the screws have a dense mount point.

I used support only touching build plate. PETG is hard to remove from inside that square.

I uploaded the Fusion 360 file as well in case anyone wants to modify it.

I originally found this on another site and used a model from there for the remix.