You can use some of the same methods as these guys are kind enough to illustrate.here
For something like those overkill "speaker door panels, you would lay out your fittfitted panel and probably make a big opening in it to access the backside of your work. Your speakers would need rings so fix them in place where you want them and stretch the fabric over the rings. Fasten the fabric down to achieve the shapes you want. For strength you can build up the backside as much as you like. Pictures are worth more, look here
, some very amateur work but combine what they are doing with the workmanship of the bicycle guys and you should be starting to get some ideas. If you plan to do door panels, remember to set them both up at the same time and do them together through each step so you can make make them as identical as possible. Very tough to completely make one and then turn around and try to make a mirror image of it starting from scratch.
I like this kind of stuff. Simple ways to make one-off parts without all that borking around with originals, molds, and stuff. Notice how nice a final finish the spandex gives versus "fleece". Though for some reason the fleece guys never quite seem to get around to completely finishing their work. You also note where they state they aren't using fiberglass at all but rather an epoxy resin mix.