All 5 double slotted extrusions need to be cut into 20.75" lengths. These will hold each row on top of eachother.
2 of the 4 single slotted extrusions also need to be cut into 20.75" lengths. These will hold the very top and bottom of the panels together. The other two remain uncut and act as a side brace/stand.
Mark the aluminum extrusions at 20.75" lengths and cut with a fine toothed hacksaw
At this point the panels will have plenty of horizontal support, but need to be braced vertically. Now orient the outer oval T-nuts vertically and slide the 2 uncut single extrusions so that they line up with the top-most horizontal extrusion. They'll stick out a bit at the bottom.
Keep in mind for the top corners to not block the IDC connectors. Place them on the second to last extrusion from the top.