I think most people use the Hammond style enclosures just because they are relatively cheap and available. I've seen builds in mint tins, and other small pre-made boxes.
Don't limit yourself though. Building your own enclosures can be fun too. Wooden boxes, plexiglass boxes, and folded sheet metal boxes are possibilities. With a little more skill you can also make welded steel boxes, tig welded aluminum boxes, copper boxes, or even cast your own aluminum, brass or bronze enclosures.
I weld a lot of my boxes from scrap metal. I have access to the tools at work, and it just seems more practical to spend the money on components when I can build the enclosures for no added cost. I'm too lazy to cast them, but I have home made tools to do that too. I have a bunch of others too, but these are some of my home built boxes. (Except the wah, although I have one half built now)