Do I even need to fiberglass?

I am wanting to add some 6X9s to my truck doors but there isn't enough room to put them in.  Someone suggested to me that i should construct fiberglass pannals for my door to put them in.  Do i even have to do this?  Are there any suggestions for what else i can do or any suggestions on how i should go about this.  thanks

if you are not sure about the use of fiberglass, you can always use wood. the use of fiberglass just gives you the ability to make it in any shape you want and blend it to other panels if desired. what you can do is take a piece of MDF, cut it in the shape of the bottom of the door, then add a ring or 2 to the front part of the panel to give the depth needed. you can always router the edges of the wood, and use some body filler or duraglass to smooth the edges