Plugging firewall holes

[MASTERBrian]

I have about 20 holes of various sizes in my firewall. I've looked at two local parts stores, a Lowes, and an Ace Hardware for rubber grommets/hole plugs. I know they make what I want/need as I am sure I've purchased them years ago for my cars. Amazon seems to be my best bet for buying a bulk and hope that what I get might work. I'm curious if there are any other great ideas out there.

I've thought of welding a few of them shut, I've also seen where another member uses JB Weld and coins to plug holes. Problem is I'm not sure if I'll want any of those for later use, when running additional wiring.

There are also two identical holes right where the tranny tunnel starts going back, they are about 5/8" and the metal is flared out....any idea what these are? I can post pic later if need be.