I clamped an end and a side to a 90° square and tacked it ...
then repeated for that side and the other end,
followed by the other side
and finally that side to the original end.
Corner-to-corner measurement comes out about the same.
Which means it is fairly rectangular ... not a parallelogram.
I measured center of each end ... then measured out 3-1/2" to either side for placement of the middle bars. Again clamping them with the square while I tacked them into place. I wanted them to fit just so, on the pivot plate.
Kind of supports that area where the wheels will be added on the outside, and makes the middle more rigid. This was all constructed with 3/4" square tubing. Now it needs to be ground smooth, then flipped over and welded on the other side.