You can use ngrok without paying, you just get weird domains like n56897as.ngrok.com (this is off the top of my head, idk how close I got to a real one). This works fine most of the time.
https://ngrok.com/docs#subdomain
edit: Just in case, I updated to 2.2.4 (newest) and it's still possible to use custom subdomains.
Tunnel session failed: Only paid plans may bind custom subdomains. Failed to bind the custom subdomain 'blahblahblah' for the account xxx