You mean the "un-sealed" underbelly. The only answer is more fasteners, learning how to drill holes in steel upside down and eye protection. Find the largest diameter stainless steel washers you can find and something soft to lay on.
Mine was so saggy I had to completely remove it from one end and reattach properly. It helps you get up close and personal with your best camping buddy!
2017 F350 Dually 6.7 Diesel
2014 Silhouette 299 RKDS
Nights Camped 2016 - 43
He who dies with the most tools, wins!