The DuckDNS.org is the most popular way to get a domain for free and it's extremely important for selfhosting.
From what I saw in sources the Cloudron has own implementation of the ACME DNS challenge. The problem is that each DNS provider has own API incompatible with others.
The Sandstorm.io instead uses the library https://github.com/therootcompany/acme.js which was also developed for a selfhosting project. The library has support of many providers including the DuckDNS.
As an alternative solution may be acme.sh which is not in JS but in plain shell instead. It has support of probably all possible DNS providers.
Interesting that the Sandstorm doesn't support setting of a custom domain but instead use it's own DDNS service sandcats.io The service is not that good and doesn't support setting of TXT or MX records.
I'm working now on writing a DDNS server that can be deployed https://github.com/yurt-page/go-ddnsd. There is already one existing called https://github.com/nsupdate-info/nsupdate.info which supports users registration. But I wish to make something more lightweight and simpler with autoregistration and I'll continue later.
You guys can try to deploy the DDNS server and autoregister new cloudron installs and give a free subdomains.
This will help a lot for selfhosters.