I have lived in the woods my enter life and have enjoyed nature all around. I have seen so many bees that have died in buckets, water puddles, and anything that has collected water that they were trying to get to. This is something that I have come up with on my 3D printer to help out some.
Create a shape and size that will work for you. In the design have the pillars in the middle stager in height from one another. That way as the water goes down new pillars breaker the waters surface giving the bees islands to land on for the water.
Getting everything together for making a mold box and setting that up. Double back Duct tape is what I put on the base to anchor the part in with. I used some plexiglass and siliconed the seams together. Once that had a good amount of time for the silicone to cure, I used a universal mold release. Letting that set for about 15 minutes. Then casting a 1 by 1 part concrete rubber molding material. I let that set for about 5 hours. Then I pulled the part from the mold.
When the mold is fully cured, I use a universal mold release again for inside the concrete rubber mold. Letting that set for about 5 minutes. Using a concrete patch repair mix is what I use next. Mixing that until it is a puddling like thickness with it. Casting that into the mold then a bit of shacking the mold to level the concrete out for a smooth bottom look, and that will help get any air bubbles out. I would let the mold set for about an hour. Then if the concrete was solid enough, I will tip it up and have a fan on it for the next 2 – 3 hours. After that amount of time it would be ready to pull from the mold.