1)Buy or make a wood plug.
2)Use sandpaper and a file to shape the plug to a snug, but not tight, fit.
3)Insert the plug into the hole of the roughly finished grip.
4)Screw a long thin screw in from the side so that it locks the plug in place. 5)Shape and finish the pistol grip with the plug installed.
If/when you want to remove it you just unscrew the long thin screw from the side and pop out the plug.