Made from scrap 1/8" ply and 1/2" ply, it has the centre of the array marked to line up with the cab centre and a stop to place the mullion edges at the correct distance in from the cab edge and also provide the right spread.
On the front edge of the top and bottom of the cab mark the centre ( measuring in from the same side to ensure accuracy )
Clamp the device into place lining up the centre marks
Glue and hold the mullions tightly into place. It's easy to see where the should line up and also easy to look down to line up the brad nailer.
This has turned a bothersome PITA task of marking and trying to hold sticks in space whilst guessing where to nail into a genuine 2 minute task from glue up to final nailing.
I have only one jig, so I set and nail both mullions at one end, then transfer the jig to the other side. Two jigs would probably work a little better but I'm not bothering to make another - the one works a treat anyhow.
Now before I get a myriad of questions saying my mullions don't look like those in the plans.....I am already aware of that.
I have mine that way to be able to use the flat part of the mullion ( a 45 deg cut ) to attach my grilles onto ( with a thin strip of carpet underneath ) 
