I was using the aluminum yard stick like cboy posted, but it was very difficult to hold and control. I suppose I could have countersunk some scews and put handles on it, but I never got that far.
I know your looking for a home made solution, but I bought a couple of AFS sanding boards and they are the best investment I've made so far for bodywork tools. They are not cheap, but the time and energy saved was well worth it to me. They have removable metal rods in them, the fewer the rods, the more flexable they get. The face is thin steel so it will flex along large curves and sand off the high spots. I have a 16" and a 27"