The corner brace design I found on Thingiverse was too weak for 25mm extrusion. After assembling the frame the extrusion could flex the plastic touching it.
I took the design into Sketchup and adding more support. I’ve added the STL to a new GitHub repository I have started using for the pieces I redesign.
The new brace design adds support on the bottom of the extrusion to allow for 3 additional mounting points. This decreases the amount of flex present in the frame.
Side shot (The bottom holes needed to be reamed a bit because I sized them too small. The other pieces are sized correctly):