Problem with NAT's on multiple devices behind firewall

This thread sums up my problem exactly, but they don't seem to think 
there is a solution which surprises me.

https://forums.freebsd.org/threads/58832/

Basically I'm trying to get some PC's and consoles to have NAT type 1 
and so far I can only get one of them working properly at any time 
without resorting to upnp.

Could use some help, thanks.