Something like a raspberry pi or equivalent, and use reverse SSH set up to connect to a server with a known address on your end.
This means that ports don’t need to be opened on their end.
Also if you go with a gateway host, shift SSH to a randomised port like 37465, and install fail2ban.
You can just use a soulseek client.
However I have a build of this daemon running on a Qnap storage device, which is super handy just for ad-hoc music searches, and people can also peruse my music library 24/7.