VeloSolo Disc Cog
If you haven't seen them
I just purchased the 15t for my MTB...
These are great cogs, well made, fit perfect, bolts included, and no lock ring
The 15t does have a wee bit of chain interference on occasion, but very very minimal.
For anyone with a disc hub that may want to try MTB fixed without building a wheel, be sure to consider one of these.
I run 32x18 free on the trail, but have found without owning a car this year, getting to the trails has been an issue, 32x18 spins you all the way there. So I bought the 15t so I could ride fixed to the trail and flip it over or punish myself once there.
32x15 on a 29er comes out to about 62gi, so not the fastest, but a hell of a lot better than 32x18, and running fixed makes it that much better.