A cradle is very useful for working on assembled instruments.  Tasks like setting sound posts are easier and less prone to damaging an instrument when the instrument is held in a cradle.

The cradle is no more than a thick piece of wood (or in this case several sheets of inexpensive plywood) hollowed on one side to fit the back of an instrument and lined with cork or leather.  Its also helpful to put something on the bottom to keep the jib from slipping on a bench.

Aside: I keep my setup tools in the old cigar box pictured below.  This are nice boxes and can be found fairly cheap.  Most cigar stores will have a pile and only charge $5 or $10 for the nicest ones, sometimes less.