Awhile ago, I built the planer sled shown in this video on Fine Woodworking's online site: Keith Rust's Planer Sled. I don't have access to the article so some of the observations below may have appeared in the article.
Voilà, my sled:
The base is a torsion box that I took care to make flat. Turns out there is a slight variance over the whole length, but not too bad.
Another important point has to do with the type of planer you have. I have a floor-standing (floor-crushing?!) Powermatic 20"planer. Unlike my Ridgid lunchbox planer, it has power rollers, not pressure feeders (after this post, I'll post about those differences). "Power" in this case really means it, unlike "power" meaning anything in "power chord" of 80's metal. Note the following picture: