I've heard of using a small spacer ring to move a fixed cog a little further from the hub flange to help a bit with chainline when doing the ghetto multispeed to fixed conversion. I know it should be metal, not nylon. Also seems like it's probably better if it's not threaded, but rather just slips over the threads on the hub so less chance that it'll screw further on to the hub allowing the cog to loosen. I don't know if there is a specific part that works well for this purpose. What is commonly used?

Use a bottom bracket spacer between the hub and cog, which come in several different thicknesses.

Perfect. Thanks!