A neck support caul is quite easy to make with a cove cut made on a tablesaw. For details on how to make a cove cut see below and/or consult any good book on joinery.
For a cove cut a block of wood is moved several times across the sawblade at an angle; the blade is set slightly higher for each pass until the channel is the right height and width.
Using clamps and a plastic tube as a clamping caul a layer of cork is glued into the channel. Under pressure, the tube, which should be about 100mm (4") in diameter, will adopt the shape of the channel.
Excerpt from the book Building Electric Guitars