Unfortunately, certmagic doesn’t have built-in support for challenge delegation right now, but I’d like to see that happen at some point.
What you could do though, in the meantime, is use the duckdns plugin, which would let you do challenge delegation:
Domains from DuckDNS are free, and you don’t need to install anything to use it.
Keep in mind though, DuckDNS only seems to support one TXT record at a time (strange limitation, but ) so if you have multiple domains to get certs for, make sure to either stagger their issuance (add each domain to your Caddyfile one at a time, with some delay in between) to make sure it’s less likely to overlap, or use multiple DuckDNS domains (there’s a limit of 5 per account, I think, unfortunately).
All that said, it would be great to have a libdns plugin for acme-dns as well if you’d like to take a crack at writing one: