I think the picture is upside down. I replaced the lower panel after a blow out. The piece was primarily held in place by just slipping it under and the lower screws near the bottom held it in place. There was a staple in the corner I couldn't see under the corner trim strip. I found it tearing out the damaged piece. Not sure how it would fall down unless screws are missing under the bottom curve.
2000 F250SD SRW 7.3 Long Bed Extended Cab Air Bags
2014 Eagle 30.5BHLT
2007 Layton 245LF (sold)
196? Shasta 16ft
Learn from others mistakes, you'll never live long enough to make them all yourself.