I'm building 4 of these, one's installed and the 3 other ones are in various stages of completition. Here's the frame and the stretchers temporarily assembled before the Dominos were machined in.

Here are the stretchers with the Festool 8/50 connectors installed.


Here the first cabinet is temporarily installed in the knee-wall while the face frames are attached to the cabinet sides and the stretchers with Domino 8/50 connectors.

The only thing remotely critical is the centerline distances between the Dominos, as I wanted all of the joints between the face frame pieces to be absolutely flush and without any spaces.
I took these standard Festool items and turned them down to a sharp point. Assembled them in the stretchers, squared them up with a small machinist square and then used them to mark the placement of the centers in the face frames...it worked well. There is a small amount of lateral adjustment with fasteners, maybe +/- .5 mm.
