Get Avg Temperature of a Fluid Subdomain?

I have defined a fluid subdomain inside a hot box of electronics.  So what's the avg air temp of the subdomain?  Just can't get started on this issue.   Can't define a Volume Goal on the subdomain.  Can't define the subdomain as a solid or feature.  Can't define a Volume Parameter on the subdomain.  Wasting time and getting nowhere.

Kevin Carpenter

SolidworksFlow Simulation