Making one is the best option; cheap to do, and takes less that an hour. A good idea is to make three of the sides out of thicker plywood, and one side and the top out of thinner stuff. This improves the volume and tone, and gives more structural strength. You can use standard metal angle strip for the corners, and fit it either internally or externally. A more craftsmanlike approach would be to joint the edges properly, but it's not really necessary. Much the same principles are used for building a cajon.