Well, if you are dead set on avoiding bellows, you could make a sliding box construction to enable you to focus (even if only to fixed points). You'd still need to make a ground glass and use it at least once, to set up the focusing positions. You'd need at least two positions - infinity, and portrait. With a 16x20, your portrait subject would essentially be life-size, so you're dealing with 1:1 reproduction ratio. Therefore, you'd have to build two equal-length segments to the box, one to position the focal plane at the focal length of the lens, and one to position it at the 1:1 position, which translates into double the focal length. This is very simple to do, but you're paying a HUGE price in weight and bulk of this camera. Once you set up the camera, you put a test subject in front, pop in your ground glass, determine the focusing distance, then attach a piece of string to the front of the camera, stretch it out to the subject's face, and put a knot in the string to mark the distance. Then you can dispense with the ground glass. To use such a monster, you'd need an old-fashioned studio stand, as a two-box (or even single box) camera of this size would not be portable, at least not by one person.
If you are set on wanting to make something like this, I'd still suggest putting on a bellows, but you can make it yourself, and to keep it simple, make it square. You'll easily cut the weight by about 2/3rds of what a two-box arrangement would weigh.