[Sapientoni]

I think I'd leave that hole open. I just revived a 2006 boxster that was "flooded" by a ham fisted owner or mechanic who had destroyed the drip pans and left them out. After I got the carpets dried out and was getting ready to install, I noticed a tiny bit of water in the floor on the driver's side. It was dripping from a rubber plugged hole in the chassis. There's a cavity in there that can fill up and retain a few litres of water. I went underneath and sure enough there were other plugs under there. I removed one of them and released the captured water. I then replaced both the pass compartment plug and the undersided plug. If there wasn't a plug, I think I'd leave it as the factory left it.
I think my 2006 cars have an ABS wire coming out of that hole.

Best of luck.