Don't forget to tighten the cog first.

Also, I am all about working with what you have (pliers, a flathead, and a hammer can do a lot on any job), but I am trying to figure out how it would even be possible to hold the wheel, line a screwdriver up, and hit it with a hammer?? I'm laughing trying to picture it actually.

If you really can't buy the tool right now, just head to Higher Gear on Fullerton and have Brooks tighten it, or Rapid Transit on North and have Bob do it for you. They have both tightened lockrings for me without too much fuss.