I would either get another piece of underbelly or some type of black plastic and cover up the "hole" with an "access panel" and screw it to the underbelly.
The dealer had to open mine to add a valve for the plumbing. I dont know exactly what they used but it looks like heavy duty black duct tape. It has held for almost two years now.
I cut a piece of coroplast 6" bigger than the cutout, install nutserts in the belly pan, then I run a bead of rubber caulk around the edge of the hatch and bolt it in place.
If I need to open up the hatch, take out the bolts and slide a scraper around the hatch and it will release.
__________________
Jim
When I was a kid I was going nowhere fast, now I'm an old man I'm going everywhere slow.
Thanks for the replies. I don't have a "hole." I just have a slit about a foot long. I chiseled out the rivets and dropped the plastic from the front to past my dump valves. I just had to make a cut about a foot from the edge to get the plastic around some wire loom that went up through a hole.