Having checked the fitting, a standard mdf ring won't work for a 6.5 inch Pioneer speaker (the screws into the car would have to go through the speaker cone!)
There is a further option - a stepped mdf ring, allowing the lower of the two to screw into the existing holes in the door, and the narrower upper ring mounting the speaker, time to measure up!
Speaker outer rim to door - 1 inch (25mm)
Hole in door - 4 inches (100mm)
Existing screws screwed further in as needed for a 9mm MDF lower ring - plenty of clearance with the window wound right down.
No clearance problems and a flat face on the door to mount the ring - all good.
No clearance problems
Speaker goes further into door than it will need to - OK there.
Outer and Inner ring both with OD of 165mm inner ring 130mm ID outer ring 145mm ID
Top to bottom - Speaker (Pioneer TS - G1712i) , Compound Ring, Door
Translucent rendering of the two rings in their positions with drillings
CAD with top, left, right and perspective views
Simplified side view showing screw positions in rings - speaker to ring, and ring to car.
Bargain MDF sheet waiting for me to cut into rings!!
Inner ring designed to be from 9mm mdf, outer ring from 12mm mdf, total thickness is 25mm inc speaker rim
I'll keep you posted, but it looks possible!