Any decent English threaded (24 tpi) bb lock ring will do and unlike some, I don't have as many issues with "suicide hubs" since I have used and still use a few of them and have had no problems over many many thousands of km of hard riding.

I just swapped out the mtb wheel I built with a fixed cog as I built myself a new wheel with a flip flop to get a second gear on Number Five and that thing must have 6000 km on it.

So..

Use industrial strength lock-tite (the red stuff) and once you set the cog and the lockring put the wheel down and leave it for 24 hours to allow the lock-tite to set up... THIS IS ESSENTIAL

You must use a brake with this set up and for commuting it should work just fine...for tricks and skids...maybe not.

The drawback to this is you cannot change your cog once it's set up and removing it may very well trash the hub... a torch is required to release the lock-tite (if it was properly done).

Ideally... a track hub is the way to go.