My old panels were very bowed. In fact they were not even in the front and rear channels, because the centers had come out so far. I suspect the absence of plastic sheeting was contributory.
The new panels appear to be too big. However, if I can get them pushed into the channels further, and I think I can with the excellent advice tendered so far, then they should fit just fine. I didn't notice any sort of angle on the edges, and just mounted them with the smooth side out. Tonight when I get home, I'll see if I got lucky and glued the vinyl on the correct side.
Your advice is greatly appreciated.
Joe