NGINX proxy_pass to dynamic port

I'm hoping someone can help me with this. I'm looking to proxy pass something like this:

http://storage.domainname.com:port 

to this:

http://192.168.12.10:port 

My problem is that the port is dynamic, and it's not feasible for me to write dozens of location blocks for it.

This is my conf (I'm using port 61788 for testing):

server { listen 61788; server_name storage.*; client_max_body_size 0; location / { include /config/nginx/proxy.conf; set $upstream_app 192.168.12.10; set $upstream_proto http; proxy_pass $upstream_proto://$upstream_app:61788; } } 

I don't think using location regex is applicable here, as the port is specified in the host. I've tried using $remote_host in proxy_pass, but that gave me a bad gateway error. Does anyone have any ideas?