Define the area you want to create the fiberglass enclosure in. Use 3M blue painters tape to mask off the carpeting etc around the area. This will protect it from the resin later. This wil be the rear wall of the sub enclosure and make the enclosure fit perfectly into your vehicle. Make sure to mask beyond the area you plan your sub enclosure to extend. If you drip resin onto your paint or carpeting it is unlikely it will come out ever.