First let me start by saying that a few months ago I hadnt ever messed with any type of fiberglassing. Since then i have made a set of Kick panel pods, a center consoule, sub enclosure, amp racks, and some transition pieces for my doors to the dash.
1. Check this site out.
www.fiberglast.com their starter kits seem expensive, but trust me once you buy the bondo brand stuff and find out you like making parts it gets way more expensive in the long run.
2. The cheapest place I have found to buy fleece is walmart. I just by what ever is on sale.
3. When attaching the fleece you can staple or use 3M vinal top adhesive.
when using the vinal top adhesive make sure to glue it past where you resin stops. Otherwise what happens is the reaction from the resin softens the glue and things start to go really, really bad trust me!
4. As far as the speaker holes go i just fleece past the edge of the appropriate speaker size hole and then sand the fleece back the hole size once its cured.
5. when spacing the rings i just cut mdf strips and then cut them to the proper length. hot glue them in place emporarliy then screw them down before stretching the fleece
6. to keep the fleece from saging leave minimal long unsupported gaps.
you can use just about anything for supports. I like to use great stuff foam. kinda messy but it works. spary in the hole then just rough sand to your shape.
hope all this helps
l8tr