JD's a whiz at this kind of thing (and kind enough to give out designs), but you should still learn some basic CAD especially if you get the itch to design other things (like your own cases) one day. I think it's the kind of thing where you can learn a lot on your own, especially with the aid of the Internet
You don't even have to pay for CAD software - qCAD or LibreCAD are free and adequate. Emachineshop and BigBlueSaw both offer their own CAD programs that come with tutorials and offer tight integration with the cutter/miller.
I leveraged a lot of the resources from the GH CAD hub thread
here. The Phantom plate files are a little out of date but they're a great starting point. There's also a lot of info as far as measurements go there.
As far as materials go, there's trade-offs between all 4. With acrylic you'll need 4-5mm of thickness or there will be noticeable flex. Aluminum will be the cheapest 1.5mm plate you can cut but you will need adequate stand-offs and/or fastening points to the rest of the case to prevent flex. Stainless/carbon steel will be the most rigid, but the most expensive to cut. If you want color in your plates, acrylic has "natural color", aluminum can be anodized, and stainless will have to be painted.
Keep in mind that one-off cutting jobs are going to be expensive, even from companies that cater to hobbyists. Measure everything twice. Compare your measurements against existing designs and against existing PCBs/plates you have. Be able to explain every discrepancy.