Giles,
I re did the door panels on my 54 Manhattan and used industrial strength Velcro I purchased on eBay.
The reupholsterer recomended using screws but I didn't want the screw heads to show,
The Velcro has held firm for almost two years now.
I believe the metal clips are available from Restoration Specialities. I found it very difficult to line the clips up with the holes in the door.
The Velcro strips come with adhesive applied so all I did was place them where needed.
Marvin