As per a post on in the main forum, I've now started to sort out derailment barriers for the boards. I found a supplier, bought two test pieces and have been able to make choices as per what to get.
The curved boards will be getting 2mm thick clear acrylic, and the straight boards 3mm acrylic. Holes have already been drilled to take bolts to mount the barriers in place.
I've placed an order for enough strips to do the ends and up to four 4'x2' boards on both the scenic and fiddleyard sides.
As far as the boards are concerned, I think the next jobs will be to fix any remaining structural issues (one or two frame elements are still loose and a couple of the top boards need regluing to the frames where they haven't sat in place properly), then to clean them up by sanding down the splintering edges, and then paint them to hopefully seal them up. I've decided against varnishing as they have too many construction marks on them. It's going to be white underside, a dark grey-blue for the sides, and cream for the tops.
Some pics of the fitting of the two clear acrylic test pieces: