Sheetrock, or similar, works pretty well. One of the best builders in our club uses it. If you cut it, be sure to tape up the edge so you don't end up with the "filling" scattered everywhere.

Before my brother started using a magnetic building system similar to http://www.airfieldmodels.com/inform...oard/index.htm he just used 24x48 acoustical ceiling tiles on a table.

I have an old desk and got a Great Planes building board http://www3.towerhobbies.com/cgi-bin...?&I=LXPF36&P=0 which is balsa boards glued together and planed flat. My board has stayed flat but I have seen posts from folks saying theirs was not flat when they got it or it did not stay flat. I've seen similar balsa boards under different brand names.

The key, no matter which top surface you choose, is to put it on a FLAT table. Use a long straight edge in multiple directions to see how flat it is and adjust as necessary. My desk is not perfectly flat but it is close. It has a low spot in the center. I used playing cards as shims and it has worked so far.