I think silverado has a point. I would expect Renzetti to produce a superior product. However, there is a current problem.
Installing a rod along the bottom that is threaded at both ends with a washer and wing nuts should do the trick for now. For even better results I would install two of them on opposite corners, along the bottom of the supports. This would alleviate the pressure on the threads if the bar and wing nut. Also instead of the wing nuts use lock nuts. The plastic would act as a vibration dampner.
It is a quick and reasonable solution till Renzetti steps there game up.