Originally had trouble with this, and couldn't get the proper amount of spacing occupancy, so I wanted to post this as a helpful note.
By placing one of the larger spacers in the front, you're able to get the notches to engage the freehub with the proper amount of overage to securely fix the entire rig together with the lockring.
I believe this is from a Deore XT 9 speed cassette.
I am running 38/17 for the snow.
You can use any cog.
I understand they have specialty single speed conversions, with special cogs, but this post is specifically to address rigging for those who've purchased this exact single speed conversion kit. Because I initially had issues with trying to use the smaller spacers up front, but then they don't engage the freehub body grooves, so it's not fixed as it wants to be when you tighten the lockring down.
By using the larger spacer in these kits up front, you get that extra body on their grooves to engage the freehub body, and lock everything down together and secure.