I came across this method using the school of necessity being the mother of all invention.
I built a swing set for my son and stupidly drilled the mounting holes very far down the inside of the crossbar tube as shown in the first image. The inner diameter of the tube is less than 70mm and far too small to fit my hand in.
I could easily reach the bolts with the end of a ring spanner but the nut would not stay in the spanner for transport. I have solved this problem in the past with a magnet but as this swing set is in the garden I used stainless steel nuts and bolts.
The solution was to tear some paper (in this case a receipt from my local cinema) and place it over the ring end of the spanner, I then pressed the nut in and the paper formed a very tight packer retaining the nut.
I could then use the ring spanner to pass the nut to the end of the bolt, turning the bolt on the outside to pick up the thread and the tightening down tears the paper and pushes some of it out, giving you your spanner back. See I told you it was easy.