Reverse Cache for Multiple Sites

[John Macleod]

I am looking to implement NGINX as a reverse proxy cache for a number
of sites, there are 6 fairly busy sites between 4 webservers.  I can
find plenty of online assistance in configuring NGINX to reverse cache
a single site from a single origin.  I appear to be struggling to find
how to configure a single NGINX installation to reverse cache multiple
sites from multiple origins.

Could anyone point me in the right direction on this, or am I overly
complicating a simple config?

Thanks in advance,

John