My FolgerTech FT5 has had diagonal frame bracing for a long time, but there was not enough rigidity in the front-rear direction. So I designed mounting brackets to fit into the 2020 rails that use common 1/2" EMT tubing as the primary bracing element. For example, UPC 091111020018 which is easily available from places such as Home Depot, Lowes and others.
A 24" length of conduit per brace will position the ends near the corners of the side openings as shown in the photos. Be sure to de-burr the ends of the conduit after cutting. The 45 degree cylindrical plug portion fits into the interior of the conduit and is 15.5mm (0.610") in diameter. It was designed to be glued into the tubing using 5-minute epoxy. Super Glue would also work, it is just a bit less forgiving time-wise during assembly.
Also, there is also a center hole that is sized to be used with 8mm or 5/16" threaded rod. You could use a threaded rod as a draw bar to pull the ends tightly against the conduit. Or you could just use the rod as the brace and it is probably rigid enough for most applications. Either method can be used to attach the ends to one another.
The mounting brackets are designed for M4 or M5 fastening bolts and t-slot nuts where they attach to the 2020 rails.
These are light weight yet very rigid and provide much-needed bracing to the printer frame. And they are inexpensive and quick to make. By adjusting the length of the conduit they can be adapted to other printers and applications as well.