I made mine out of 1" plywood by cutting six pieces 14" square and glueing/screwing two together with the top grain 90 degrees from the other. I then gave them a good soaking in sealer and two coats of white enamel paint. I then drilled a 1" hole in one of the corners so that I can use my awning rod to place and remove them. For those of you who are wondering why only six pieces....I only have three jacks.
I got one of those heavy duty grocery bags that all three fit in nicely and I usually use them when on any surface other than concrete.
Ron