I use a peice of tool granite. You can order them from various too places like Grizzly etc.. that's been polished down - up to really big sizes but I use a 24" x 36" piece which is big enough for my stuff ifyou build the wings in halves.

Sure it's really straight but I use it for other stuff and it's really heavy (which helps in the - oops I bumped the table area as well).

Why not start as straight as possible so when the monokote does put in a slight warp - it's not warp added to already warped structure?