Similar to the above, I'm sure it would work fine without a cover, but personally for larger holes I usually make sheetmetal caps (and usually get fancy and play with the bead roller on larger ones), paint them and then screw or rivet them in before I cover em up. After that I have used the butyl backed sound deadening stuff and yes, it works fantastic. And does a good job on smaller holes. I do that as I figure dust/dirt/grime might work its way underneath the surrounding area and maybe cause other issues down the road, although... I'm probably worrying about nothing

1949 Chevrolet 3/4 Ton - Still Solid. Regular Driver
OT Vehicles:
1925 Hudson Brougham Super Six
1950 Chevrolet Styline (Parts)
1952 Canuck Pontiac Sedan Delivery (Well Underway)
1973 F250 4x4 Highboy
1977 F250 4x4 Lowboy