Next came the fiddly bit - assembling and installing two walls, solo, in sub-freezing unconditioned space,,,
I had to remove the fixtures out of the way, clear the whole space, then build the back wall in the standard manner, laid out on the floor. The wall has to run right between the plumbing and drain risers, it just fits. There will be an access passage behind this wall for future service, hopefully never needed.
The left side wall was not as easy due to existing ductwork in the way; I had to "stick build" it, one board at a time. Done.
Everything fits as it should, and connecting it all up should be a straightforward endeavor. Here you can see that the serpentine cut did not result in loss of structural area of the deck; it's out of the traffic lane and the partition wall will have decking with a joist under it.
I will still need to build the partition walls, then sheetrock everything with moisture-resistant sheetrock, then install the doors and cabinets, etc...