In our stock of wood is some LitePly about 3mm thick. More expensive than normal ply, it has the advantage of being cutable with a heavy craft knife. Since I wanted the top to follow the contour of the hill, this seemed worth it. Besides, I wasn't looking at big chunks of the stuff - and anyway, there's no point in buying these materials if you keep saving them "for a special job".
Anyway, the hill had to be trimmed back to allow the ply to be fitted, but I clamped it in place with a wodge of filler behind it and then tidied up the top (yes, I should have cone this before working on the mud) for a neat job.
It looks stupidly good. Eventually, I'll paint it grey. Matching the colour of the rest of the boards won't happen so a contrast will look a lot better.
As well as looking good, the ply will protect the polystyrene from knocks as the model is moved around in the future - another win!