I needed some horns for a project using some geared dc motors from adafruit that are in a micro servo form factor.
https://www.adafruit.com/products/2941

They are 20mm in diameter
14mm diameter on center spacing for the mounting holes.
sized for M2 screws.

Update 5/5/2016: Added a V2. V2 Added a small fillet on the top and bottom outer edges. Makes it easier to sit it in the omni wheel.