Rad, that is a unique setup. I've found the u-channel, but nothing on the other piece. Was this a mod you did? Or if by a previous owner, any chanse you can reach back and ask where they got said pieces? I'll keep searching, cause I'm that kinda gal, so if you find something please let us know.
I've found these clips work to hold the plastic onto the folding walls. This may not be an option for you, but just wanted to toss it out.
https://www.amazon.com/Tablecloth-Cl...s%2C267&sr=8-8