This first piece was to make sure my measurements were correct.
So far so good.
Next will be to build another frame that will fit on top, to the top of the sliding glass door, then add a skin (1/4″ ply), cut out a square and insert the Amazon sourced plastic cat door.
Update: January 2nd 2024
Except for a coat of paint, cat door is done. 🙂
Just need some weather stripping, foam to plug up the gap created when the glass doors are not in the closed position.