The #1 building tool I have is a true "flat" building surface. Don't kid yourself...very few things are flat. If you can push a pin in it...it's not flat. Nothing celeous like ceiling tile or grey Homasote board. Nothing wood like demensional lumber or any door. The best of the best is a piece of pool table slate. You want a piece of Italian slate (very dark and known for not being brittle) sized for a regulation table which will be about 1" thick and about 32" x 51". It will weigh every bit of 250 pounds. Check first with billard table repair people or billard table suppliers and they will usually have pieces of unuseable slate chipped in handling. You should be able to pick up a piece for less than $25.00...maybe even free if you are a young, attractive, female having a pretty smile and happen to be wearing a short skirt and/or low cut blouse! BTW, it is best to put a piece of 1" or 2" foam rubber underneath it so the slate "floats" on top of the foam eliminating asymmetrical sharp point loads.