In the past I have just stuck a penny over them with some silicon on it.
I have installed some/uninstalled some things going the hole in the firewall above the pedal area on my '73. Question now is, what is best material to plug gap in the hole while allowing future access? Gob of fiberglass insulation (using gloves of course!)?
rubber plugs
hot dog down a hallway ?Pennies are cheaper. Just one cent per hole. Speaking of holes, I knew a gal once who.......................