Simon,
The way I've seen it done looked pretty straight forward. You can mock it up from topside using very thin metal sheets. You need to pay more attention to not marring the underside though. Because once you get it pretty much like you want it (you'll have to ignore the step up from the existing fiberglass to the metal when looking at the right side), you will turn it upside down, apply mold release to the metal underside: apply wax or spray it with PVA. I prefer PVA but be careful not to get it on the existing fibreglass. It is alcohol/water soluble, so clean up isn't too bad. Which one to use? wax gives a slicker finish, but you tend to get more missed spots which can be a pain in the arse when it sticks. PVA tends to leave a matte or semi-gloss look, but tends to relase much better. (Less is more with PVA also: don't apply any one coat to heavy or it can sag and give you an ugly finish. If you need to smooth transitions or create fillets, use some bondo and get it as smooth as possible before applying your mold release. Fillets will also make the release much easier. Most of this is straight and flat making it much easier. Only the floors of the nostrils has much curve.
But, do be careful of compound curves that can lock the backing mold in place. Some areas will have to be given fillets afterwards to look smooth; getting crisp outside corners can be another pain in the arse because the matt will want to raise up and bubble rather than staying flat against the outside 90º. One way to deal with this is to have your main wall sheets come just to the edge. Take some 2-3" strips of cloth, fold it lenght ways and iron it so that the two sides lay against one another. You can now use these to strengthen and build up the corners. (Just like ouside corners on dry wall.) Now, lay up your glass and resin on this; giving yourself a pretty good overlap once you've built up to even with the underside of the existing glass. One trick here is to scallop or zigzag the existing fiberglass edge: /\/\/\/\/\. Cut interlocking zigzags to match with your cloth. It give vastly more surface area to bond with and gives a very strong joint. To take it a step farther, you can taper the thickness of the existing points and make the matching valleys in the cloth smaller and smaller until the layer which matches the surface of the existing glass. Either way, continue to lay up till you get your desired thickness. It is a good idea to also make each successive layer over lap at least 1/2" more than the previous layer for 2-3 layers above the "existing surface" layer. This will make the join less visible and give it some backing. Once it's setup, remove the metal backing molds and clean up the seams, sanding and filling as neccessary.
A couple of hints: When ever I am going to add onto existing fiberglass, it is a good idea to roughen it up to give it some tooth (60-80 grit) and I always scrub it real good with acetone before any resin hits it. You probably want to do this before you put in your metal molds for the final time before layup begins, but be sure to wear latex gloves from then on so as not to get oils from your skin on the clean glass. It is a good idea to wipe down with acetone just before you go to work on it, just-in-case; also keep in mind the note above about cleaning any off any mold release also. Using great care, try to squegee (sp?) out any excess resin. This will keep it light and, believe it or not, stronger. If you have issues with strings raveling off your cut cloth, either get lock stitch woven cloth or, if you are like me, get a good, reasonably priced tight weave and go over the cut edges quickly with a bernz-o-matic or similar LP torch. This will weld the weave and help prevent raveling strings which I hate with a passion.
HTH,
Lynn
