The clamps I drew up for you won't do anything for making the corners 90 degrees. It will only help hold the cross members perpendicular to the rails and keep the cross member from moving while tightening the screws. I was assuming that there are bolts going through the side beam into the ends of the cross members. That may not be your case?

Ahren will fix you up. Waiting for the parts will be worth it.

CarveOne