This is a simple device to support the chuck whilst it is being fitted to the spindle flange. With an 80 mm chuck it is just about possible to hold the chuck and get the spindle nuts fitted to secure it to the spindle. When I upgraded to the larger 100 mm chuck this became almost impossible. For while I used to clamp a piece of 20 mm tube in the chuck and then this could be located in the spindle to support the chuck whilst the spindle nuts were located and tightened. However, this simple cradle is much easier to use.
The cradle is made from a block of wood as the base. The upright section is a piece of MDF shaped to a radius of 50 mm to support the chuck. This is screwed and glued to the base.
This shows the underside. A piece of 6 mm plywood is glued and pinned to the base so that the front edge is clear of the carriage guide.