I spent money on solid core doors (bought two), but neither one of them was as flat as I would have liked, so this is what I build on:
http://www.factoryglassdirect.com/p-...1in-bevel.aspx
On top of it lays sheet metal for magnets.
This may be an overkill and it is not the cheapest solution around (although really not too bad), but you can be certain it will not warp over time!