I made one out of a few pieces of steel pipe, a Pony pipe clamp, and a couple blocks of wood, contoured to fit the seatpost. I clamp it in my bench vise when I need to use it, and it works great. If you don't have a bench vise, you could mount a pipe flange to your workbench. If you don't have a workbench, you could use the same principle to make a floor model. If someone is both interested and confused, I can shoot a picture.
For simple procedures like oiling a chain or a quick der. adjustment, I often just stick the saddle nose through a loop of nylon webbing hanging from a garage joist.