If you're going to go to the trouble of getting a carry case for your Nintendo 3DS, you may as well go all out and get this hitman-esque case from Japanese accessory-makers Actec.
It's going for almost $150, which given the size seems a bit much (even taking into account the cost blowout converting USD into Yen). But if you want to step onto a train and convince people you're about to go work, and not just go somewhere to play a bunch of handheld video games, it's also one of your only options!
Aside from, well, just sticking a 3DS inside a regular briefcase. But where's the fun in that.