Ian, I have used these from CPC in the past for an AML EZ1073 and an Oakley filter, case is 1.2mm steel and the front panel is 3mm aluminium... despite the rear panel being steel I have put XLRs, jacks and an IEC/switch in them and don't remember it being too much hassle.I wish someone would do a nice steel 19 inch case with aluminium front and back panels - they are so much easier to woodwork with than steel.
CPC doesn't have any good technical documentation on them for some reason, but look at Penn Elcom's site here at the Q&As and they state that if you send them the necessary design, they will custom cut the rear panel for you, not sure on cost though. For some reason it's cheaper to buy them from CPC than PE directly but apprently they price match so, maybe worth enquiring
Only things I didn't like on these was: the venting in the top panel—not for any real reason other than aesthetic preference, but if you were doing anything with valves you might want them to breathe anyway! And minimum 220mm depth, they felt a bit oversized (for my needs at least) but if you like a lot of space then again, not really a concern.