All recommendations are welcome. In particular how to vibrate a two part mold without it breaking apart. I did not weigh the mix and water, but i suspect i used too much water. Also added about 2% plasticizer.
I want to make a drain where this puddle forms, but stock drains from hardwarestores expects more difference in the ground levels. I only have a few centimeters to work with to a french drain around the corner, so i want the outlet to be on the side close to the surface. I have a drain grate that will fit on top of this thing.