How to attach fiberglass?

Ok this might be a stupid question.. but here goes anyway lol

Here is my plan.. I have a 2001 PT Cruiser. The PT has a shelf in the back. So, im gonna make a fiberglass box, and put that on the shelf. But, I want the entire hatchback to be lined in fiberglass as well..so I am then going to tape the entire rest of the hatchback, and floor.. masking tape.. resin.. etc etc. Question is.. I am going to tape up to the rubber molding. Do I just stop there?.. or do I somehow have to make a lip and tuck it under the rubber molding for that clean look?. Also, when I get the mold for the sides, after I paint the mold.. how do I attach it to the walls of the hatchback?. I would assume screwing directly into the plastic isnt an option. How is that done?. Also, will that create alot of rattle when 3 15"s are pumping?.

about the rattling,  the best way to stop any system from rattling is by not turning it on

if it is some serious thumping, it will rattle

its going to rattle, 2-12's running at 200w each was enough to rattle my civic (and thats after sound damping)

so with 3-15's running high wattage, you're goin to need some serious damping

dynamat everything you can reach in the back end of that cruiser, and i would also recommed using some expanding foam if you can open some internal panelling

you'll want to be careful with the expanding foam, especially on the hatch.  i used expanding foam without sealing off the trunk latch and it was very difficult getting everything out from there.  so if you're going to go the expanding foam route, just make sure it will not interfere with the mechanics of the hatch latch