An MDF baffle is very simple to make. You can either use 1/2 or 3/4 MDF, depending on the height of your speakers. 1/2 is probably better as it will not cause interference w/ the inside of the door panels.
You can use the factory plastic speakers as a template and re-use factory self-tap mounting screws to hold it in place. I highly recommend using some sound damp (dynamat, cascade) around the speaker mounting hole. Also, you will want to seal the baffle to the door w/ some type of gasketing material...no rattles, etc.
If you really want to bring some life and definition to your midbass, use some sounddamp on the inner door skin and put cascade deflex pads behind the speakers over the sound damp. At high volumes, the sound wave from the rear of the cone will bounce against the inside of the door panel and back to the speaker causing muddiness, distortion, etc. The deflex pads will alleviate most of this.
Let me know if you need pixs (I can't post them but maybe you can)