How thick is the steel? I built my magnetic board on the top of a hollow core door. The door is what keeps the steel flat for building on. I build my table with 18 guage steel. You can see more of my building board in this build thread that I did:
[link=http://www.rcuniverse.com/forum/m_3439708/tm.htm]Pay It Forward build thread!! LT-40 build.[/link]

Hope this helps

Ken