> Out of curiosity, wouldn't it be easier/cheaper/faster to use some existing off-the-shelf design and drill/machine some holes if you need it slightly modified?
For small production runs, definitely. There are standard aluminum extrusions for boxes with PC boards. You slide the PC board in, and provide custom flat end plates with holes for connectors and controls. Here's one of mine.[1] The end plates were cut on a laser cutter. Here's a supplier in China.[2] For small boxes, the aluminum extrusion alone should cost a few dollars.
Somewhere above a few thousand, custom injection molding becomes cheaper. Amusingly, these are better boxes than plastic injection molding, but don't look like consumer products.
[1] https://github.com/John-Nagle/ttyloopdriver/raw/master/board...
[2] https://www.alibaba.com/product-detail/CHANGHE-electronics-a...