Ok, let's see if I can make this clear
Baffle= basically anything a speaker is mounted to, take a piece of plywood, a whole sheet, one half sheet, one quarter sheet, etc, cut a hole in it and mount the speaker to it, it is then a baffle. A speaker box is made up of sides, top and bottom and a front and a rear baffle or some consider the rear just a back and not a baffle.
The door mounting location is another form of baffle only this time in steel and in some cases the speaker is molded onto a plastic frame that is bolted to the door, the frame then would be the baffle.
Take your plywood above, substitute MDF.
MDF= Medium Density Fiberboard, made up of glued and pressed saw dust basically, very dense, very stable, easy to work with, used extensively in speaker building due to non resonate properties, low cost, etc, etc.
The STI we just worked on, as an example, needed new baffles to space the speakers out a bit further from the door metal so the magnets(motor structure) could clear the windows when they are down.
I removed the stock speakers and the plastic, sq face pop in mounting tabs the stock screws mounted in to hold the speakers in the doors. I just need them out of the way.
Then I took a piece of paper and made a paper rubbing of the area that was flat and transfered this to a piece of 3/4" MDF, marked it out, cut out the center for the speaker hole, cut out the shape I did the rubbing of, sanded it down a bit and test fit it to the door and made sure the windows would go up and down fully. (acutally I skipped that part because I had taken carefull measurements before and knew I needed 5/8" spacers so 3/4" were going to be fine.)
I reveresed the design for the other side and made one more then painted them black to seal up the wood and make them hide better when installed.
Basically we just substitute the stock plastic mounts molding around the stock speakers for a piece of wood, MDF, that now is the mounting baffle for the new speakers
FYI, Even though we seal up the doors the best we can, they would still be considered Infinate Baffle speaker systems because of the air leaks they will always have, car speakers are build for this.
I hope this answers your questions, I search and do not seem to have any good baffle pics at the moment.