I build on a conference table, dead flat. I then bought the balsa building boards from great planes. They are dead flat too.
I did have some of the Guillows balsa boards and they lasted about 5 years before they developed a warp. To use them, I just clamp/weight them down and they are flat again.

Don

PS: I have the Cloud Bound 4 still in the box...way too many projects ahead of it. I think I really need to pare down my kit list...I'll be dead (twice) before I get to them all.