You got it, it is nothing more than a bushing that the front bearing housing sits in. It forces the motor to only move on the axis of the crank, hence no thrust changes from the soft mount. If you think about it, to reallly get any benifit from a soft mount it must be soft enough for the engine to move, but we only want movement along the axis of the crank. The unwanted soft mount movement will casue thrust changes. A nose ring fixes the "naughty" bits from soft mounts.

You can make your own from any suitable stock, such as ply, phenolic, metal, etc. You can use fuel tubing as the bushing. You can also buy them comerically made, but you will need to be sure they will fit snugly to the bearing housing on your engine. You will also need to be sure the nose of your plane can handle the nose ring load. Any plane with a removable cowl will present problems as the cowl won't be strong enough to handle the ring loads.

http://www.centralhobbies.com/engines/nose_rings.html