I'm not sure if u are willing to put the time (and stand the fumes that will stay in your car for a few weeks), but i have a custom made fiberglass box, and it's really light considering it's size. All you need is fiberglass, and fiberhair bodyfiller (to stop air leaks), and you'll be set. Took me about 5 days, and i still have majority of my trunk since it's totally molded in. If u need help with making one, i can give some advice.
