
The first picture shows the headstock being held against a beveled wooden block by a large rubber band with a foam spacer in between the headstock and block. The block itself is held by a small vise. When the ukulele is flipped over to work on the other side, the bevel block is reversed.

The second picture shows two stiff pieces of packing foam, shaped to fit the waist of the ukulele. These in turn are held against the ukulele with a long adjustable slide clamp. The foam is slightly compressible and provides a firm cushion for the sides.
aloha,
Dave Hurd
http://www.ukuleles.com