also plan on removing metal to allow for adequate airflow into the door panel. the woofers need plenty of airspace - more than what the pods will provide. i agree your best bet is another wooden piece to outline the point where the mounting pod attaches to the door. a few dowel rods glued in place will allow you to position the mounting baffle as desired. that way you have a piece you can glass from a workbench and not on the panel - then cut out the panel, secure the pod to the panel (leave enough door panel material to allow you to screw them together. then seal, smooth, sand, and finish. are you going to paint or cover the entire panel?
with the additional weight of the new pods, you'll want to ensure the panel stays put and doesn't vibrate - additional screws or attachment points may be prudent.