Another good option for enclosing a small-board system would be an aluminium enclosure of the type used for guitar effects pedals. (Of course, a digital guitar pedal is an SBC in an aluminium box...) They're seriously robust (you can, of course, happily stand on one), they can quite easily be drilled to hold I/O and power ports and control interfaces wherever you want to put them, they can be painted or printed on in a variety of ways, they're available in a wide range of shapes and sizes and you can get them very cheap. See for example https://www.mammothelectronics.com/collections/enclosures (just as an illustration: I don't know enough about this particular supplier to endorse them).
For home hacking, I'd rather not use a conductive material for my enclosure - too many ways to release the magic smoke.
If I were making a large number of things that I needed to be that durable, I'd be happy to engineer all the standoffs and isolation properly, but frankly wood and plastic are just way easier for throwing something together in my garage.
Great enclosures. The all-metal design is an important property for guitar pedals, it acts as a noise shield. For the same reason, they're probably not the greatest choice for radio circuits.
Did you have good conductive contact between the lid and the body? This is important and needs to be over a larger area, not just points (otherwise you might have an antenna).
You can get special conductive tape (very beautiful btw, made from eg copper and conductive glue) and cover all the slits. This works better if you have conductive contact to the box, but still helps if not (in this case get the cheaper tape with non-conductive glue).
But as people involved in this space say, EMI is voodoo and I suspect praying to the EMI god, putting little metal amulets on all the cables and making strange markings with this conductive tape are all necessary to limit emissions below the limits. Homebrew electronics are the worst, and passing EMI tests with them can be a special kind of hell. </rant>
Radio waves will penetrate into a Faraday cage. How much they penetrate is a function of the cage's conductivity, holes size, waves frequency and the phase of the Moon. It also changes if you ground the cage, and if you are smiling or frowning at the moment.
Aluminium is great but sometimes expensive depending on the application and the market: one can purchase an aluminium pipe/rod/whatever at a hardware store for a lot less than a stomp box enclosure made with the same amount of the material and sold online. It's a niche market where a couple sellers can easily fix the prices.
Fortunately there's enough demand from DIY and small-business effects makers that a good selection of different Hammond-like boxes is cheap and readily available online even as single units, at least if you're willing to drill your own apertures. Maybe USD6-USD12 or so per box isn't near the unit cost but it's low enough not to worry about until you need at least a few ten of them. If you can accept something lighter but less stompable you can also get similar plastic enclosures for even less from electronics suppliers.
Yes, there are lots of off the shelf enclosures. I've been using Hammond aluminum extrusions with laser-cut black acrylic end plates to fit the connectors. The result is a nice black box.[1]
Many Arduino/Rasperry Pi type boards have connectors on every edge. It's hard to box those things. That's what motivates these two-plate "cases". If all the connectors are on one edge, it's easier to package the unit.
