I made mine out of 3/16" aluminum diamond plate, and they have a handle so they're easy to set & retrieve. I think you're on the right track with the size, using 12" x 12". Many use plywood or 2 x 12 boards. Some drill holes so they can retrieve the blocks with their awning hook. Camping World used to sell black plastic pads, about 9" x 9" with metal flip handles (but I haven't seen them in their latest catalog). They work well, are lightweight, and pretty easy to clean.
Last Brave 2004 34D