HAHA! Chris has a solution for all your various needs.
Just make wooden cabinets, not too big. It really depends on the application. If they are small household speakers to listen to in your living room, you need not really worry about having the box of adequate size and not too big/small. But if you are using them to present sounds to an audience, that is when you want to go with specific plans. As for a sub, I wouldnt build your own without knowing good dimensions. If it's a powerful sub and you make the box too small or dont put the right airholes in it or whatnot, you could easily blow the box up.
Talk to Chris, apparently he's the man around here for sound stuff. He is a DJ after all...