It is a little hard to tell with your pic on the position of the lockdown strap on the left hand side; It looks like it is sitting past the slot in the receiver but I cannot see if it is sitting as far past as the nearest strap that is "one slot" out of alignment. I would think if it is out of square then at least one of the lockdown straps would be hanging in the slot cockeyed. If it is out of square that web pulling trick looks good.
If it turns out the face plate is bowed, there two options.
The first option is rather drastic. You would have to make a series of kerf cuts.
Something like these guys are doing before they put up dry wall.
The 2nd option will not be so drastic but you would have to remove some wood. You will need some wood working skills.
1) You would have to raise your play field and mark a pencil line along the bottom edge of the receiver bar.
2) Remove the receiver bar.
3) Depending on how neat you want the job to be you will need some clamps and a 2 x 4 and clamp the 2 x4 under the line you marked at the lower edge of the receiver.
4) You will need and angle sander and will need to sand away the pushed in area of the face plate until your receiver will sit flush. The clamped 2 x 4 will keep you somewhat along a straight line.
5) It will not be noticeable when the lock down bar in installed.
If it was a solid piece of wood you could clamp it and steam it into submission. But it is plywood. 964fe44f85ab975e536bec0014bf35f440d8aed6 (resized).jpg