Singlespeed & Fixed Gear - Slippage and a part question

I have a part question for y'all. I wasn't able to get my lockring to fit tight enough to secure my cog without any slippage so I placed a plastic/nylon spacer from a 7-speed cassette in between the lockring and the cog and it worked fine for a while. Now I've started skidding a lot and applying more backpedal action and the slipping has started again. I'm assuming this is due to the compression of the spacer. Any thoughts on a part that would replace this spacer, something metal that wouldn't compress. Any idea what this part is?

might be a good idea to get a better (thicker) cog instead.

What kind of hub/cog/lockring combo are you running?
How far are you able to thread the lockring on with that shim in there? I dunno, I, personally, would try and find a combo that works without needing spacers. A shim may work, but if you're skidding a lot, that last thing that you want is to strip out your frankensteined cog/hub sometime you really need to stop. If the hub and cog have been slipping, I'd check to make sure the threads aren't already messed up.
If you're really set on finding a shim, I'd just start trolling around hardware/junk stores. It depends how thick you need, but would grinding the teeth off an old cog work?

Some details of your hub and cog would help. My first thought is to find either a wider cog or use a BB lockring as a spacer.

I'm using a Van Dessel Hub with a 14T unknown brand 1/8 cog. And the lockring is unkown as well. The hub has two different diameter threads, the first and widest for the cog and the second smaller for the lockring. "What I think is happening is that the width of the first diameter thread portion is slightly wider than the width of the cog, so when I placed the lockring on there it would only tighten up until the larger diameter portion and then stop. With this set up I got about 3/8 pedal turn of slippage. I then placed the spacer in there, which has the larger diameter opening and with that in between the cog and lockring I was able to tighten the lockring down to where there was no slippage. With increased backpedaling and skidding it has started slipping again.

I think a wider cog would come in handy, I'd like to go from 36 14 to 42 16 anyways.

With a 14 tooth cog your options are limited. A spacer might get in the way of the chain and that would be bad. I would recomend the Dura ace (or something similar )16 tooth cog if you can. My DA cog is fairly wide and should work well on a Van Dessel.