We're going to need a lot more information to help here, I'm afraid
- How many channels do you want (one/mono or two/stereo would be the options, I presume)?
- Are you looking for a single full-range driver (per channel, if more than one channel), or to make a two- or three-way speaker? (My advice would be to make it two-way.)
- Where/how do you intend to use it (indoor or outdoor, desired listening level, etc.)?
- Does the briefcase need to also hold other things, or will its only function be as a speaker enclosure?
And lastly, why do you want to do this? (Not that it's a bad idea if your goal is aesthetic or similar, but it's just not the optimal way to do things if your goal is sound quality.)