1. make a wooden pattern of the opening you want to achieve. i would recommend MDF. remember to use proper breathing and eye protection as MDF is very nasty to your lungs when it is sanded or cut.
1.1 test fit the screen into the wood opening.
1.2 shape the face of the wooden opening to roughly what you want the final profile to look like. you can take off a bit more that you intend to, since you are going to cover it with fiberglass.
1.3 secure the pattern in the opening. you can use wood or plastic bonded to the bezel cover from you car. basically you want to rigidly secure the wooden screen opening to the dash bezel.
(note: i have specified wood to ensure you can secrew the screen into it. if you want to re-use the plastic case from the screen you can do that too.)
2. use a piece of fabric to stretch across the gaps between the wooden opening and the bezel. make it tight, as this will dictate the basic shape of the fiberglass work. use a fleece type material, something that will soak up the fiberglassing resin.
2.1 secure the fabric to the wood and the bezel. make sure it is tight.
2.2 apply a coat of the resin to the cloth. follow the directions. let it dry. this will be the base for you to do your remaining fiberglassing.
3. see the sticky thread relating to fiberglassing work. there is a nice tutorial posted that may be of some assistance to you.
4. dont blame me if you screw up. i have never done this before. just read about it.
5. sand the surface of the fiberglass and reapply materials as need to attain the surface shape you want.
6. repeat sanding and filling as needed. i believe you can use bondo or some other surface filling material if you want to fill small gaps or air bubbles on the surface.
(note: you WILL have to paint the entire bezel. the fiberglass will be some color that does not match the bezel. just warning you that you will have to paint the entire thing to make it look uniform. or you can cover it with a vinyl or leather material.)
best of luck
~mike
edit...
here is the
fiberglass thread