A shadow board is simply a sheet of material held at 45 degrees to the bed of the lathe. A shape can then be drawn directly onto the board or a sheet of paper clipped to it to make it more reusable.

To use them you draw your shape on the board or paper and then shine a light centrally on the front side of the wood being turned. This makes the shadow on the paper. You then turn the wood, removing the material while observing the shadow on the paper., stopping when the shadow lines up with the  design on the shadow board.

You can find the sketchup file for this design in the 3D warehouse at the following link. There are a few more additions to be made, but the majority is there and usable. Sketchup Page.

The Shadow board consists of the board itself and two supports, these are clamped onto the lathe using nut and bolt shown in the following images.