I took the couch back/upper bunk off my Scamp
13 also. I simply used a 1/4" x 3" piece of oak the same length as a ledge and screwed it on using the holes that were left from the couch back hinge.
I now have a nice little shelf for odds and ends. Speaking of ends, I did not put anything on the ends and to date nothing has fallen off. It's amazing the stuff I put on it from coins, to Sony CD Speakers, to pens/pencils, to keys etc.
I have pillows that I place against the wall and I've not noticed a problem with the shelf hitting me in the back. Without the pillows I could not be comfortable sitting on the wide couch leaning against the front wall of the trailer as my feet would dangle.