I am going to show you how I made a concrete block. This block is my prototype. You can see the broken down mold in the background.
This block is about 18 inches long x 10 inches at the base x 11 inches tall. It has a foam core inside, the walls are about 2 inches thick. It used one 80 pound bag of redi-mix concrete. So it doesn't weigh that much.
This block was a lot of work, I had to cut a bunch of foam into triangles and then tape them together to form the core. It also has a wire cage inside of it too. That was such a pain to form out of 6 x 6 wire grid.
The next block will be more like a concrete flower pot, and will sit 'upside down' with the opening on the bottom. I plan on stacking these on top of each other, with two on the bottom and a third one on top.
This was made out an old real estate sign that was destroyed by vandels, so don't just go stealing signs. I had permission to use this sign.
Step 1: make ends
I started with a flat plywood panel.
Then I added one end and braced it with a 2 x 4.
I wanted a ledge on the top so I added a piece of wood. notice that I tapered the edge of the board, to create some draft on that part of the mold. I'm not sure if that was needed to aid in the removal of the block from the mold, but I worked in the plastic injection molding industry, so I know that molds need to have a draft.
I then added the other end.