Ben, I'm sure you know this, but you don't have an option other than cutting the panel at least a little bit. The hole in the panel and the stock speakers never lined up from the factory. I don't understand why they did this, but that's how it is.
I had the same problem, but I fixed it in a little more exotic-than-normal fashion. It basically involves removing the door panel, filling the cavity with a 2-part expanding urethane foam, machining it to fit, and mounting the speaker on the outside of the panel. The foam adds an amazing amount of strength to the panel.
I don't know if you have a big gap like that, though. Looks like the panel is pretty close to the door. Maybe reinforce the panel with a piece of tempered hardboard or plywood, and then install the speaker on the outside.
Or, you could go the route of cutting out the panel to match the speaker's diameter, and then permanently mounting the speaker grille to the door in that spot so it automatically covers the speaker.
None of this prolly makes sense when typing it out. Pictures would be helpful, but I have none. Sorry.
Oh, and Bud, it's either MDF or Particle board that you would want to use.