Poco's on the money - I move the skin about 1-1/2" on my birds (1-1/2lbers) steering to a place where there's no blood vessels. When I'm done, I let everything snap back into place. I've never lost a bird once they were off the table. I think the time they are tied down is more stressful than the operation itself. The time spent suturing would be better spent with the bird recuperating.
I suggest pulling the skin toward the tail before making the incision. That way the skin covers the wound when the rib dilator is removed, so there is no need to suture. Spray with vetericyn before putting the capon in his recovery cage with food and water.