So up to now you've got no money invested.

What do your dropouts look like? If they are vertical, you are going to need some kind of slack take up device but that kind of negates the single speed panache. . The simplest way to do that is to just keep the existing derailleur in place.

If you have horizontal dropouts, you could try to disassemble your existing cassette and find or fabricate some spacers to make the cog that you like line up with your chainring. I'd look to see if I could find some PVC plumbing that had about the right ID. It doesn't have to be perfect because the free hub body will hold your cog concentric. You'll need to keep the smallest rear cog because it has ridges to match up with your cassette lock ring.

Good luck.