Fiberglass enclosure inside of door

I am looking to build a small enclosure inside of the rear doors of my 02 ranger for a set of 8"s.  I want to leave the factory door panel in place so that the door looks stock.  There is a small storage compartment that can be removed to accomodate mounting a sub.  I have made measurements and I know I have enough space for an image dynamics v.3 or punch series 2, but I don't know how to go about creating a sealed enclosure that will stand up to water and whatnot.

Here is what I am thinking.  Start by deadening the hell out of the door.  Then make templates that will form the side walls of the enclosure. Then cut those out of MDF and screw/glass them to the door panel.  I am not sure how much sheet metal I can cut away.  Is there anything I should be doing or looking out for?

Use MDF anywhere there will be a flat surface. Then come in with fiberglass for the curves or odd shapes. Try not to cut away too much sheet metal because that could lower the strength of the door.

I'd probobly use the Image Dynamics woofer because it can go in pretty small enclosures.

I would certainly use the I/D woofers.  My buddy  competes in the s.q. for usaci and he used 2 10" I/D's in the back factory location of an explorer sport that usually holds 5x7's. it came out killer and sounded great.

Any metal hey cut out he just rebraced with some flat steel