You can plug the drill hole with a sheet metal screw, or a bolt of a size near enough to the hole size that it will threaded into the hole without needing a nut on it.
Or, yes, use a nut on the bolt. But put the bolt in so that the nut is on the outside. That way if it loosens, you can hold the bolt with pliers and tighten the nut with a wrench. Then there's no need to get into the pipe - which you'd have to do if the nut were on the inside.
Last edited by Sunny Boy
on Wed Jan 11, 2017 4:06 pm, edited 2 times in total.