for any hole .5'' or under i just welded them closed while holding a piece of brass on the back side
weld won't stick to the brass and the brass acts as a heat sink to prevent weld blow out
a piece of flattened copper pipe works well too, it just gets really hot quickly
for bigger holes i just welded scrap 16 ga squares, carpet covers them well
i shaved the firewall flange while i was at it