I wish there was a standard...but there isn't.
For my system, I cut a "clearance" hole in the sub panel and reinforced it with aluminum angle, as specified by Vans. In this hole I built a rack that will hold the Dual EFII ECUs, GTR20 com, GAD29, and GEA24, and PLX F/R Amp.
I am in the process of wiring the GEA24 now...there are LOTS of wires...