On my machine (approx 2'x3') I bolted a 3/4" sheet of marine ply to the table top, and surfaced it.
Into this sheet I also cut a grid of holes, so I can use bolts through to the table top (which has tapped holes).
I then take a 1/8" sheet of hardboard/MDF and cut a matching grid of holes. It's this sheet that gets used as a spoilboard - as it gets bolted flat whenever I bolt parts to the table.
The surfaced top doesn't warp, as it's bolted down, and replacing the spoilboard is trivial (because replacing it doesn't involve doing a surfacing op).