The "hole" is there for a purpose, at least on Glocks.
It allows water and debris to drain / fall out of the action lock-work through the smaller hole in the top of the opening in the grip.
Of course, some would argue it also allows water & debris to fall into the action.
But if those types of guns needed a plug, Gaston Glock would have put one there in the first place. Glock Perfection and all, don't cha know!
I'd leave it open as designed..
rc