I don't know about making it stronger, but if you just want to stop the rattle try plastic drinking straws. You can purchase a pack of them at almost any store and just make a slit down one side of each straw you use. Place two on the bottom and slide it into position, then place one on each end and position the end caps, then place two more on the top of the panel and place the top cap on. It should fit tight and rid you of the irritating rattle.
It's not permanent and you can always try an alternative method.
It's worth a try at least. Good luck.
- RCT