05 dodge ram custom box

I have an 05 Dodge Ram 2500 Quad cab truck and am wanting to make a custom fiberglass speaker box for under the rear seat.  With all the curves that are in the opening, i do not want to use MDF for the box and am needing to know what i can do to make a mold of the openings.  I have thought about using poster board and foam to attempt to make a mold of the openings under the seat....  Is this possible??  or is there an easier way....  I am planning on using the fiberglass for the bottom curved portions and then attaching that to MDF for the top of the box as it will just be straight

Is there another way for me to make a "mold" of the curved surface under the seat???  

Here is what I am building for my '03 Ram:

https://www.the12volt.com/installbay/forum_posts.asp~TID~93682~PN~1