I rode my first fixie for several years with the exact same setup as yours and have friends that still do. You need to thread the cog on very tight and use blue locktite - use a chain whip to apply torque. Then use loctite on the lockring and tighten it as tight as you can, using the proper BB wrench. Let the loctite set up and you are ready to ride. It will stand up to braking forces if you do it right. Also clean all the threads as clean as possible with a solvent that leaves no residue before assembling with loctite. Contamination will ruin the bond. Give it a try before you spend money. Like I said it worked for me - never loosened ever. My new fixie has a Surley flip flop hub with real left handed lock rings.