This is me making one with an orbital sander that lost its sanding pad.
For this you'll need:
Cork, or some pad
Pipe hanger, sometimes called strapping
From the pictures it's easy to see the construction. Strap the sander to to the board. The biggest thing to keep in mind is not strapping it down too tight. If you fix the base of the sander to the board, then it's the sander that will move. Notice the 3 screws on each side of the strapping in the final picture, this adjusts the pressure that the sander is applying to the wood.
Just place your mold on the rest of the scrap and it will work the bubbles out of the concrete. For small molds I think this may have been overkill, or too powerful. I had to pulse the sander to keep from spraying stuff out of the molds. I'm not sure, but I think there was cavitation happening. I don't know if it was my poor quality molds, or to much vibration. Larger molds had no problem. Watching the liquefaction in effect was fascinating!
Next weekend I'll unmold the things I made today and update this. Till then...Happy St. Paddy's Day