Try your strap clamp diagonally across the long axis, (wrapping a clamp around the cabinet will do nothing as you need to make the diagonals equal) either at the legs or put 2 legs on upside down on long diagonal and pull with clamp till cabinet square. Putting legs upside down allows strap clamp something to attach to just above cabinet opening, you may need to fill between legs and cabinet with shims to fill any gaps.