What you have pictured is what I use regularly. I've made 4 of them at different times and upgraded a bit each time. 25# is plenty! I don't like using my 50# that I made first, it's too heavy. I also add a short piece of steel conduit sticking out parallel to the ground a couple inches from the bottom of the bucket and set it in the concrete with the 2x4. It becomes the bottom handle and the 2x4 becomes the top handle. Makes it much easier to move even after the bucket itself is long gone.
Any other thoughts after you made a few of them?