This particular test fixture was created with a Funhouse/BOP style display, a pre DCS sound board, no fliptronics support and no DMD support.
It was modified along the way. Generally, the CPU and driver board stay in the same place. The display driver for the BOP era display goes to the left of the CPU. Mine has the DMD driver above the standoffs for the BoP era display driver. And I swap out fliptronics and sound boards as needed.
DCS sound boards don't exactly fit right, so you have to get a little creative. I keep a couple chunks of foam and cardboard around to keep boards from shorting on each other if I pack it 'full'.
Most test fixtures that evolved over time suffered a similar fate. If you look at a WMS 3-6 fixture that was later modified for Sys7 support, an external board with displays and controller and partial harness was added. My sys9 fixture had the displays for P1 and P2 swapped out for A/N because I use it primarily for Sys11, and I added a backboard with Sys9 boards as well as dual 16 digit displays like ES uses, and there's space on the backboard for a DMD and controller for DE style CPU boards.
A tech has to be flexible.
I have some ideas for modifying mine in mind, but right now I am working on fixing boards and building out a new workshop, so stuff that works but is a little cumbersome just has to stay for that way in the meantime.