If you're at all worried about things not fitting as planned, go to a sign or plastics shop and have them cut a panel from 3/16" plastic of some kind. Should be more than durable enough for test-fittings. Most of them can work with the same DXF file you would take to a metal-cutter.
I used Front Panel Express (http://www.frontpanelexpress.com/). Their software is free and once its done you can send it off to them to get cut on the CNC. It even has a pricing feature as you go long.
As for the visual part, I printed out accurate size pictures of everything (most manufacturers have these published already). and then placed on the panel for the initial fitting.
You could also use a piece of clear acrylic plastic from Home Depot. Cut the holes with a fly cutter or Dremel and add parts to check for a rough fit.
This is good to get the 3D clearance behind the panel. Then you get your own true "glass panel"...
Gill,
Seeing this makes me think about making my panel from plexi permanently. Of course, it makes it a lot more important to make the wiring and plumbing pretty.