Belly Pan being uncooperative

I had to drop 2 of my belly pans to address a water leak (2006 KZ Montrego Bay 5th wheel) . The pans are the black plastic sheets which were held in place by screws on the cross pieces of the frame. However, the left and right sides go into a channel/groove, perhaps one inch wide, on either side. I managed to get the panels screwed back in place in the original screw holes (no easy feat); but, the panels are putting up a fight when I try to get the side edges back into the groove/channel. For now, as we are leaving town for a long weekend, I have zip tied the sides at various places, as they were hanging down on the sides a few inches.

What's the secret to getting them to go back into the grooves?

Thanks
Ron S