I suggest using 2 inch steel angle that is 1/4 inch thick for the frame of the bend jig. The end bolts also need more beef. Here should be two 3/8 inch bolts per end to further enhance stiffness. Bolts with all thread and 3 nuts each will allow adjustment of the jig opening.
The original design will flex and ruin your flats big time unless that jig is clamped with numerous large C-clamps and even pinched in a large vise.
To help "guide" the flat into the jig some 1 ¼ x 1/2 inch bar stock is useful. The economy of the cheaper materials for the jig is instantly lost with the first ruined flat.
VD in AZ