I just made a bench with a Maple solid core door. I flush mounted it on one end and put a 3 inch overhang on the long sides and over hung the other end enough to accomidate a router. I would guess its about 18" and you cant tell its not secured any less undernieth than the parts that are. And i didnt waste a piece 0f 3/4 ply under it. i screwed the door right to the 1x6 top plates.
If a pretty picture of an eagle or a skydiver and a witty quote is all it takes to motivate you, you probably have an easy job....the kind robots will be doing soon.