I've seen some 120s with integrated sleepers, turned into conversions. Mostly what I've seen was some people who used the factory flare of the cab, then cut it off and made their own corners, finishing the back of the cab off flat, to mate to a box.
You just have to buck aluminum rivets. We have a rivet hammer in our air chisel, and you hold a chunk of steel to the back of the rivet, and pound it tight. No pop rivets. Our box was actually off of a straight truck and had a roll-up rear door, so we built our own rear panel and bought pre-painted aluminum panels, buck-riveting the panel on, to appear stock.
You could also use metal bonding adhesive. We glued all of the gray skirt panels along the bottom of the coach with chemical bonding adhesive, same type of stuff they use to glue roof panels to cars.
Wild Times Motorsports