Rethinking the Baseboard

I've built my railroad on a special purpose table built in my basement. Unfortunately with all this rain the plywood I have been using has warped and I can see bulges and swellings all over the place.

I considered using Foamcore board but it is very expensive and, lo and behold, it warps as well. Check this out:

That's 5mm foamcore. Of course the thicker foamcore uses less card vs foam so it is far less inclined to warp. However, it's expensive - about $13/A2 sheet.

So I'm left with heavier plywood or...how about just plain polystyrene sheets?

I noted some on eBay for $15 each but of enormous dimension - 2400 X 1200 X 25mm.

Yes, 2 of these will just about do it. One for the baseboard and one for the scenery, ramps etc.