Some get them welded up by body shops, some use a fibreglass sheet offcut & metallic filler to fill them, some use rubber plugs... some just bog them up & complain when they fall out when the boot shuts for the first time. Lots of ways to do it. Not all turn out quite as nicely as others, and none as nicely as having a non-drilled boot (but welding can come very close if done properly).