One method is to fabricate and install a freeze plug. You need to fabricate the plug using stock of the correct thickness or slightly thicker than the spar flange and skin stack-up. The plug can be made by hand with some patience, or you can have a machine shop cut them for you. Make sure the plug fits very snugly in the hole, you may want to use liquid nitrogen or dry ice to shrink the plug, but in my experience a thin small diameter aluminum plug does not change much dimensionally when frozen. After the plug is fit, you will want to fabricate a doubler plate, or strap to retain the filler plug from the back side. I would at least use two rivets on each side, and one rivet in the center of the plug to secure it to the strap or doubler.
