Mike,
If you have bar clamps (not too long) or quick clamps (the ones where you squeeze the handle to actuate the clamp), you may try lightly clamping the ones you've put in to prevent them from moving, otherwise, put a spring clamp in front and behind the form on each side to hold it in place - you'll be clamping the hull, but the side of the clamp will be right up against the form.

Gero