Be careful not to bend it too far or too fast but, I'm sure you're aware of that already
I plan on cutting a wedge shape starting from the 'back' of the seat/tube, identical and adjacent to each other(same place/shape, but different sides). Remove the wedge piece... grab top of seat back and pull back to desired angle... weld back into place. Thatll make a 'sharp' angle at the sides, but a gradual angle on the plate, that 'can' be formed into the similar 'shape' of the back side of the bench seat, w/ alil hammering and bracing(body work, or panel fabrication, simularities...lol) I 'may' be able to make it identical, then all Id have to do is mount a cushion at the upper back, area then an angled base/butt plate(probably w/ the same hole as on the bench seats... the postierior bone/cheeks are 'cradled' in the hole as one sits on it. The padding is more so to 'form' the shape of appearance of the seat(giving it depth), instead of cushioning.