sub enclosure on Liberty roof

I'm just getting into custom fiberglass work and have tried to get ideas together on what I want done in my vehicle (Jeep Liberty).  I need the cargo space so I was thinking of putting the sub enclosures on the roof at the back of the Jeep.  I don't know if that's a good idea so if anybody has a suggestion on that let me know!  Also, if it is a good idea should I lay the first layer of glass over the upholstrey (after doing the tape and tinfoil prep)? 

This will be my first project by the way so any help would be appreciated.  I'm basically doing this on everything I've learned on the internet for the past month and a half.  I saw a post with this weblink for a tutorial and it has some good info!  http://web.njit.edu/~cas1383/proj/main/

You know that sounds like a good space saving idea.  I think you may want to think about a few things though:

2.  How thick will you make this enclosure & how heavy will it be? 

Honestly, I think it can be done but you may have to plan this out pretty good.  Im sure we will have more input/comment/suggestions in this topic pretty soon to help you along the way.