Door frame core profiles can be joined with "Pallet Protector angles" as outer splints.(They come in 2 sizes - see pic ) Plastic edge strip off cuts (shown in white in pic below) are inserted on the inside. Core profiles are usually straight cut at ends, but shown 45 degrees in pic
4 x 6mm BOLTS Plastic edge strip off cuts are attached with another nut for cross bracing, For corner uprights, the other bolts can have Plastic edge strips going at 90 degrees to join adjacent upright.
E. Full view of parts used (white edge strip is not shown, but also used inside core profile for inner tension strength)