They're made of steel, so are reliably opaque, are very durable, and usually have very tight fitting but easy-to-open lids that might be light-tight right off the bat.
They come in both rectangular shapes like tea tins, or low cylinders like cookie tins. (See the previous episode on cylinders for special issues about the effect of curved backs on the image.) Altoids tins are great but they're very flat and therefore extremely wide angle. (See the next post about issues with extremely wide angle images)
Of course, they're decorated on the outside, but the inside is shiny metal, so they need to be given the flat black Krylon treatment.
I've had this tin around the house with various sorts of small bits stored in it for forty years. It originally held some hard candies which we bought from a neighbor girl on a school fund raising project.
I decided to put the pinhole in the top and the paper in the bottom but if the tin is really light-tight it shouldn't matter. I suppose there's the argument that if it's the sun shining into that joint between the lid and box that's the big problem, putting the pinhole on the side means the sun will always be shining on the top and therefore be less of a problem. But then you'd have to make sure you carried it around that way, and that benefit goes away when you decide to shoot a vertical.
This tin been pretty well used so the top does not fit quite so tightly anymore, and I got some distinct fogging on my first shot.
I normally avoid camera designs where you have to tape them closed with opaque tape every time you load them. It strikes me as wasteful of tape, is difficult to do under safelight conditions, and often damages your camera when it's removed. In a workshop where the objective is to get the participants to make just one negative each, it might be workable.
But in the case of a metal box, it might not be so bad. The tape doesn't really have to be applied until you go out into the sun, so you could do it in room light. It peels off the metal cleanly and if done carefully that piece of tape could be reused. I usually warn people against using cheap electrical tape because it tends to come unstuck at the most inconvenient moments and your camera just falls apart, but for this purpose it's not really holding the camera together, it sticks better to metal than to cardstock, and that's how I handled this problem. I only took three exposures but used the same piece of tape and I wasn't being that careful, and I could have used it again.
One issue I haven't mentioned is that containers, including cylinders and cardboard boxes, come in a variety of shapes, but photographic paper comes in 8x10 sheets which evenly divide into 4x5, 4x10, or 5x8 sheets. (Of course paper comes in larger sizes, but they're going to have their own even divisions). If your container is not one of those sizes, you're going to have to trim the paper under safelight. Paper cutters are usually the most convenient, and scissors can be workable, but it does add a little element of danger using sharp implements in a darkroom. One thing I've done is make a template out of dark card stock that I could hold to the corner of the sheet so I didn't have to be able to see markings on the cutter bed or somehow mark it for cutting with scissors. And there's going to be some waste, which generally bugs me. If you're also making contact print positives, though, these scraps could be used for test exposures.
It might seem tempting to put the pinhole directly through the steel, but it's awfully hard and getting an accurately sized and round pinhole is very difficult. It's usually better to drill a quarter inch hole and make the pinhole in some sheet brass or beverage can aluminum.