On the Extra there's no balsa back there (on the bottom of the fuselage). I just cut the MonoKote. But if you examine the photos closely you can detect a balsa stick across the front of the opening between the stringers under the covering. I slipped that in and glued it into position after I cut the hole, then ironed the MonoKote down to it. That way, the covering is supported and you can simply iron a patch into place. If you ever need to adjust the amount of weight you can remove the patch, then replace.