David, thanks for the warning, will certainly heed that.
At the moment, they will probably be able to fall just through, because they are the same thickness top or bottom.

Kirt, I think I saw a picture describing what you said.

I big bungee going from the hole to the front beam,
so the board is some kind of jammed in the slot.
I think I'll try that (with the rope )

Thanks for the support.

Michael