More ideas from Francisco
The important thing is that these two holes are concentric and they meet in the middle of the block. Use a drill press, drilling the larger diameter first and then following on with the smaller one

This is probably the simplest and quickest router jig you will ever make, but it is a certainly very effective way of making your own dowels. You won't need much in the way of materials to make it, though the router does have to be mounted under a