Found this:

http://www.mjmautoinnovations.com/speakeradapters.htm

Thoughts? I guess one problem would be that they are not "tall" enough, so a gap would exist between the speaker and the door panel's speaker gasket/grill area.

I know I could go with MDF, but I dislike the idea of putting wood into my door panels :-\

 

Just use some spraypaint to seal the wood from the elements.