Most of those free dns settups use subdomains already. Unless you cant, for a temporary domain name, I recommend just creating an account at a mainstream/reputable dns provider and buying whatever is on sale for a dollar or two per year. Dont plan on renewing because those ones shoot up.
Hopefully the .self tld becomes a thing because it is supposed to be free: https://hccf.onmy.cloud/2026/06/21/reclaiming-our-digital-selves-hccfs-vision-for-a-human-centered-top-level-domain/