Ride your bike up a steep hill close to your house (to tighten the cog) and then cruise back to your garage without putting backpressure on the pedals, then use the lockring tool to tighten the lockring. You shouldn't need to hold the cog with a chainwhip while tightening the lockring. I've never held the cog while tightening my lockring and haven't had any problems with cogs loosening.