I did fill those with dicor. I did a zig zag pattern of dicor along the both inside faces of the trim piece. I wrapped the corner all the way across with eternabond and sealed the edges with caulking to prevent (theoretically) any water that might get past the caulking from flowing back into the luan / mesh. Not my idea. If you look at the side trim, you'll see the eternabond wrapping the side corners. Why they didn't do it on the bottom, probably a Friday afternoon job. The underside screws go into 1 x 1 x 1/16 wall box tubing. Where most of the screw heads rotted off, I used a larger size screw, ie go to a #12 instead of a #10. Probably rotted the tubing around the hole. On the front slide which appeared a bit worse, I drilled additional holes, but you want to use the base drill size, not the clearance hole size, or the screws won't bite. Or you could use self tapping stainless screws. McMaster Carr part 90415A460. The factory uses crappy steel ones. Btw, my rig is a 2017.
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