There are some places where you can get free domain names that work with DNS-01. Here is an example of one: dnsapi · acmesh-official/acme.sh Wiki · GitHub. Otherwise cheap domain names are only like 4$ per year.
But back to the original question:
If I need a quick and temporary webserver to serve static files I do the following:
- ssh into my TrueNAS using my user
cdinto the directory whose files I wish to share- run
python -m http.server 8080(in a tmux session if I need the webserver a bit longer)