A fully parametric ring blank made in Fusion360. I couldn't find one, so I made it (people selling stuff like this for 7$ on various sites: what are you smoking?).

You can change pretty much everything: hole size, wall thickness, top ring diameter, how far the top surface sticks out, the taper etc. 

Open the source file in Fusion360 and change the parameters, the relevant ones are added to favorites, the others that start with "z_" are just calculated values, don't mess with those.

Enjoy!