depending on your car, one tip i can give is this.
when you make your baffle, cut the top and bottom at angles to avoid last minute jigsaw problems. also, cut it about 1\4 inch too small for your hole. put masking tape or sound deadener all the way around where your baffle will be installed to create a barrier for the adhesive. the adhesive will go into the gap but not be permanently attached to the car due to the barrier of tape\deadener. then use some of that black can great-stuff spray foam. it cures rock hard, but a little goes a long way. you are better off laying down a single spray of the stuff until it hardens rather than spraying tons at once. it does not like to stick to itself. i used liquid nails around the baffle in my saturn, and i wish i had known about expanding foam. that is a lesson learned.
so get your baffle as close to the size of the trunk opening as possible and use the foam to seal the edges. in the NS15 install, i used the stock trunk floor carpet for the back side of the IB to match the carpet walls around the sides perfectly. and then i made a new false floor out of wood and carpeted it black. and then i put Grey on the front to match the interior.
really, the biggest thing to do is seal up the trunk as best as possible. i would recommend NOT using the black great-stuff spray foam for sealing holes in the rear deck and things of that nature to avoid flexing of panels during expansion.
post a build log and ill be sure to throw you some tips. ive done three so far, and i plan on doing more in the future //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif