Ecky's is out there, I know I've seen it, but here are some others as well (dedicated audio threads, so quick and easy to find):
https://www.insightcentral.net/forum...adget-way.html
https://www.insightcentral.net/forum...ers-ideas.html
Basically, MDF, plastic, or even a solid Plywood would be fine for making the spacer rings (heavier/denser the better). Have to cut your door panel's plastic "cup" out. Mount the ring and speaker to the metal door skin and seal every hole the best you can (probably replace the plastic liner, if you still have it.). I like to use aluminum duct tape myself. Deadener if you can, that always helps. If you can manage to seal your door speaker to your plastic door panel as well, it should give further improvement (but that would take some work).
Edit: YES here's Ecky's in another thread!
https://www.insightcentral.net/forum...ml#post1390457