Red,
I think the simplest would be to run a piece of pipe through the center of your form so it sticks out about 6 inches or so from both ends. Keeping it centered in the form while you pour in the concrete might be done by sealing one end with a wooden disk and then using smaller pieces of wood to hold the "axle" centered in the open end. After the concrete cures you would then need to bend up a frame the runs from both ends around the roller to a tow bar arrangement. You could use roller bearings on the axle to make it roll easier. If you look at the pictures from the links above I think you can see what I am saying. Where in Texas are you? I live off of Lake Conroe and have a farm in East Texas.
Hope this helps,
Tom