I do this for a living and have been for years (and currently go to school to be in the medical field)
First off, I hope youre GF isnt around FB with a face mask and a tank top? lol.
Fleece is a GREAT fabric to mold using FG. Its easy to work with, absorbs alot of the FB and also cheap. Try to use good bondo and take your time sanding and prepping the piece. Nothing sucks more than having done all that work then having the putty crack after it dries. PREP PREP PREP
Decide what size juice bottle youre going to be using. Maybe for asthetics, you can use two smaller ones and the amp(s) along the middle in an angle facing the rear of the car. What i'd do is get plexy glass, fog it and use the spare tire compartment as the center point of the install and making a false floor out of the fogged plexi. LED's underneath will give it a nice subtle glow.
You using Fusion products? Good luck and feel free to hit me up with any questions.