/diy/ - Do It Yourself

I know you said you tried steel, but I would make a new form out of steel. Make it really smooth and polished and chamfer the corners nicely. And make it so it unpins at the corners. Put it on a sheet of metal or something slick and flat for the bottom surface, or hell if you want it rough you could put it right on the ground. If you did a slick surface like melamine or a sheet of steel you could have a way of indexing something simple for the center hole. A 2l soda bottle or a plastic coffee can that is firm yet squishy enough to squeeze in and remove from the hole once the concrete has set up.

This way you unpin the sides and simply knock them away from the concrete to remove it.

Sides could be made from simple flat stock steel in whatever width you want the final block thickness to be.

Each corner "hinge" should be made so one part has one center hinge barrel and the other corresponding side has two. Do this the same all the way around so they can be pinned together in any configuration and still be correct.