BTW...if you happen to live in Seattle, I would be happy to let you borrow a 15t for free.
Cogs aren't expensive either...these are good for the money.
https://www.retro-gression.com/colle...dard-track-cog

If you have a rear wheel that's fixed on both sides, you can always run a 15t on one side and a 16t on the other. That way you can flip the wheel if you want to gear up/down (or a 15t on one side and a 14t on the other side).