I have recently made a kit that allows you to adjust the cast and height of a shotgun comb (i think they are also used on rifles). You can buy similar kits from Graco in the USA but once you include shipping and possible customs charges it gets a bit expensive.
The kit consists of a base, top part, M6 bolt, M6 washer, M6 nut, and 2XM5 grub screws. The base and top part are machined from aluminium, while the other parts are stainless steel.
The base has a 25mm diameter, and is 8mm deep.
The top part has a 20mm diameter and is 10mm deep.
Complete kit.
The kit