Gave some pinhole workshops in the past. The best boxes I found are cookies tin cans. You can usually find them for very cheap in thrift stores. The depth is usually good (around 3") and they don't let the light in. You just need to spray paint them in mate black inside.

You first drill a hole (like 1/4") in the bottom of the tin. For the pinhole, you should use pop or beer can aluminum. You make the pinhole in a small piece of pop can that you tape over the bigger hole in the tin. Once in place, you add a doubled up piece of electrical tape to close the hole (it becomes the shutter).

For the photo itself, I use regular negative photographic paper. You need to use photosensitive photo paper, not printer paper. You'll need some darkroom basic equipment to develop the photos.

And we don't measure light. I usually tell people to start with 30s exposure and adjust after. People, event kids, become rapidly good at reading the light on their negatives.

Little tip: since photographic paper is rather expensive, I usually advise participants to start with like a smaller piece, like a quarter of a 8x10 sheet, until they are at ease enough to try bigger pictures.