I took mine down once to route some 4AWG battery lines... Lots of self tapping screws for me... literally 100s of them haha...
After that I kinda of know now where things are located so the time time I needed to get inside I just took a few self tapping screws down. After the third time of needing to run battery cables I installed a 2-inch PVC line with easy access points between my 12VDC power distribution center and the battery switches locations..
Cutting a three side flap doesn't sound like a bad idea as well... I'm sure you could come up with a quick seal if you went this route...
I park my trailer right next to the woods here in CAMP BACKYARD so its pretty important for me to keep the whole underbelly material in place. Lots of field mice and squirrels here...
Roy Ken