I have used this mounting adapter to mount JBL Club 622 (6.5 inch/ 16.5 cm) speakers into the front doors of my Renault Megane 3 (2013). The adapter is from 3 rivets (120 deg) to 4 screw holes (90 deg). There are two speaker positions, 15 deg right or left offset from the original Renault speaker position.

Note; This mount is made to be used with the provided JBL screw brackets (circled in pictures). Also, I have put holes for 5 mm rivets instead of Renault's 6 mm -> Use of a 5 mm washer on the rivet is required (from the back, or drill to 6 mm)! A riveting tool with a head of a diameter of 21 mm or less must be used else it will not fit in the hole. Lastly, make sure that the screws which hold the speaker do not stick out past the adapter because there are no holes in the door at those positions.

I have added pictures with important dimensions marked in case you need to make your own for a different speaker set, unfortunately I cannot share the solidworks file.

WARNING to avoid rainwater leak: I recommend using self-adhesive weatherstrip on both sides of the adapter to avoid rainwater leaking into the cabin. Use weatherstrip between the adapter and the door (glue to bottom side of adapter). And between the adapter and the speaker (glue to bottom side of speaker). I recommend you cover the entire surface of the adapter with weatherstrip (2 rows). Also, I recommend printing at 100% or close to 100% infill because 3D printed plastic is generally not water tight at low infill values.

Other information: In my own build I found that the amplifier Pioneer GM-D1004 fits in the dash where Renault's BOSE package amplifier would be. It is possible to mount the Pioneer GM-D1004 to the top of the glovebox using self-adhesive VALCRO. The wiring harness from the radio to the amplifier's "high level inputs" is available on amazon, but make sure to buy the correct one! The Megane 3 has different radios with different connectors! My own Megane 3 (Phase 3, 2013) has the TomTom RLink Live with voice recognition and everything is still fully functional after installing this amplifier with the shown wiring harness (pictures).
Note that, because the amplifier has a "high level inputs" input voltage of 10V it will max out at about 60% volume on the radio, because the radio outputs voltages higher than that (Battery voltage > 14V). Going above that 60% on the dial you will experience severe clipping/static. It is possible to avoid this by replacing 4x 500 ohm SMD input resistors on the amplifier PCB with 11k ohm (or more, personally I've put 20k ohm). The resistors to be replaced are located near the operational amplifiers and are in series with the 22k ohm input resistors. One per operational amplifier. They should be easy to find using a multi-meter. Alternatively you can also add an external resistor divider. Please spare yourself the headache and only attempt to modify the amplifier PCB if you know what you are doing and have the equipment required for soldering SMD components of size 0805... The Pioneer GM-D1004 comes in multiple PCB revisions.
As a final side note: It is possible to operate the JBL speaker's - tweeter's +3dB switch with a small allen key trough the speaker grill.

If you found this "Thing" useful please consider tipping. I wish you the best of luck with your project.