Here's a pic of my mounting bolts:
I just had the panel laid onto the door when I drilled the holes.. that way those studs go right through the holes ready to get their nuts put on.
Also, instead of fleece, use something more like spandex (but a little thicker). Fleece makes the object loose some of its shape and also sucks a lot of resin. You stretch the fabric, coat it with resin, then let it cure. Once it's hard you take your fiberglass mat and start wetting layers of it onto the hard shell you created. For audio panels I'd do 2 or 3 layers prolly.