It was apparent from the outset that getting two boards to line up well on exhibition hall floors is tricky enough, adding two pieces of 6mm mdf a foot above this and expecting it to line up in two planes would be nothing short of miraculous. So as the post a little further down shows, two 43" lengths were cut and arranged to sit a fag-paper away from each other. As expected, due to inconsistencies in the boards and gallows units , they didn't quite sit level or in line. So in order to mask and align, a keystone was constructed from two pieces of shaped 6mm mdf with another piece an inch deep sandwiched in between held with wood glue and a lot of pins driven in from both sides. This simply clips over and is held by its own weight.
All that's needed now is to paint and maybe engrave PRR on it.