Atlas made and Atlas and Sears sold a total of six QCGB models. The majority if the internal parts are the same in all six. But there are four different main casting part numbers. The two models (three if you count the 1700 which came with a template for drilling a hole for the tumbler pivot) for the 10" replace the FWD-OFF-REV gear box. It's mounting bolts are vertical and the main casting sits on top of the bed. The three 12" models all replace the bearing housing that supports the left end of the lead screw and the short shaft that the screw gear mounted on. The attachment bolts are horizontal and go into the side of the bed instead of the top (except for the one additional one added to late production of the final version). Plus all of the 12" change gear models already have the tumbler assembly, which was an addition to the 10". With enough money to throw at it, you can make just about anything fit. But in my opinion, it isn't practical to install a 12" box on a 10" lathe.
As to why no one makes an aftermarket box, there were I think two being made when Atlas finally made one available from the factory. They pretty soon went out of business and then Atlas quit making the 10". There just isn't enough of a market to keep a company in business.