Fill holes in Electrical Panel

How can holes in Electrical Panels be filled/blocked?
I have come across panels in which cables were not passed through knockouts, instead a larger single hole was made to pass three cables. Now, cables glands or knockout plugs won’t work there. Any ideas?

There are knockout seals made to fill the unused holes.

If the hole is just too big for the connector reducing washers can be used.

Purchased at any hardware store.

Hire a qualified electrical contractor and they will know how to plug the ko’s and meet code compliance.