Do a quick search online for “electrical ko plugs” (knock out plugs) there is quite a range of styles and sizes (from 1/2” up to 4” at least)that will solve any hole infills you might have very easily. In the floor pan I might put a little caulking so they are water tight.
Once you get over 4” look for “clean out wall plates”, they get really big and are stainless or chrome, a 3” clean out wall plate is close to 6” diameter, you just need to make a bar to span the hole to screw them into.
Pop in
Wing nut tigthten
Center Screw tighten (my fav)
Etc
