I came across this idea at the home-de-pot when I found in the cement section plastic forms for making walkways. They did not have a form that I liked or one that was big enough so I decided to make my own.
I built a simple mold with 1/4" X 3" poplar stock from the local big box hardware store. Then I used 1/4"x 1/4" square stock to make a decorative pattern. The whole thing was held together with grabber screws. I used Vaseline as a release.