I took a suggestion from Dave Platt and purchased a 4'X8' piece of 3/8" tempered glass for a work bench top. It was not cheap, but will last a life time, is perfectly flat, you can glue jigs to it and then just scrape them off with a razor blade. You can tape your plans underneath between the supports and build on top of them, I also use it as a light table to copy plans, and (depending on the size) you can build two or three planes a once on it.