As far as size of oscillations go I think I would just have made a wood frame for that mold and attached a plywood tab to bolt a cheap chinese pad-sander to...

That said, a plywood T on a pad-sander is also likely to be quite effective and much simpler, or even simpler just a piece of plywood that is 50% longer than the pad(and the bit you stick down into the concrete could also be used to spread it out...)....