I’m a bit worried from what I’ve read, both in the past, that using UPnP can make your network less secure.
I’d like to help the community by allowing inbound connections from other nodes but not at the expense of security.
Could I not just use some fixed port-forwarding rules instead?
Yes, you can manually set inbound ports.
The default inbound port for BTG is 8338. (For Bitcoin, it’s 8333.) On your router, point TCP 8338 to the computer running your BTG node, and just enable inbound on the node.
If you’re also running a testnet node (which is low-load and can run at the same time), you need to add port 18338.
Note: you can also set a non-standard inbound port in your node’s bitcoingold.conf file if you want, but be aware that you’re likely to get extremely few inbound connections if you do. By default, a node will not try to connect to a non-standard port unless it fails to find eight outbound connections on the standard port.