I bought the little cube tool with the nubs on it, and had to make some modifications. The surface with the right spacing had four nubs and I only needed two, so I ground off one diagonal pair. After that it could get a good grip on the two slots in the piston.
I then proceeded to break the piston loose and turn it with the tool - many times, and it did not seem to go in. I finally went back to the standard C-clamp, and found that as I turned the clamp screw it got a grip on the (now loose) piston face, and rotated the piston and pushed it in at the same time.
I think the best tool would be a C-clamp with the nub tool on the face of it so that it got a very good grip on the piston. This would allow you to turn the piston and put pressure on it at the same time. However, even the Ford tool shown in my shop manual seems to rely on hand pressure to push in the piston.