Hi Micheal,
I will measure up the verticals, the next time I'm in the clubrooom and let you know what the dimensions are, as they were arrived at by guestimate. When I had the layout up on it's legs, I decided to cut the backscene down as it was way too high.
The layout is portable and so a one piece beam will preclude taking the layout down. Having given it a bit of thought, I may use a a couple of timber blocks with an EM gauge society, board alighning dowel, or failing that another toggle clip on the inside front to hide it.