why do you clamp/mold it at all? If you can use a plaster knife with any dexterity, just apply it liberally in the area to build up material, and sand (36-40 grit) and file it down to shape. Alternatively, if you are replacing like a missing chunk, you could just back the area with cardboard and tape to give you something to 'pile it on'...I've done this literally hundreds of times.