Situation: Linux box (Debian/ARM) behind a Draytek router which will variously connect to the Internet via ADSL, WiFi, 3G, etc.

I want to have SSH access to the box whenever it is online without relying on having a routable or static IP at the router, ie the box will need to make and maintain an outbound connection to me so that I can access it when needed.

I think this is what Chris does on his boat, but any pointers to the right way to go about this appreciated. Not looking for a full VPN (not least because some 3G providers block VPN access). The Draytek is already setup to manage office-to-office VPN but it's the failure of that which has prompted this.