I carefully unscrewed the end of the diaphragm mount on the arm of the Victrola. I made some measurements and came up with a plan.
Here’s the basic design in two parts:
The lower part is to redirect the sound from the iPhone speaker (on the bottom) down through Victrola. It has a slot cut in along it for the iPhone to fit in and a hole that will line up with the Victrola sound path. The ends of this part are plugged up with dowel to send the sound down through the hole and not out the ends. Check out the diagram for some dimensions.
The upper part is just to support the iPhone so it doesn’t swivel down to the turn table. So it just needs a slot cut along one side of it for the top of the iPhone to fit in.