I cut a hole in a box, put a belt through that hole and attached it to my pants. I also put candy inside the box for added effect. The suit part I already had. Its a pretty simple, easy and fun costume. Its important that, to make the box fit right, your pants might have to be unzipped (still buttoned though). Another thing to make sure you do is to hide the belt inside your pants to make it more realistic.
(If you only plan on putting your junk inside the box, I highly advise against it. If you do, it will not be very comfortable at all!)