You can also FG over polystyrene foam patterns with Epoxy glass. I've done it with RC plane parts. It's one off per pattern.
You make the pattern as nice as you want and take into consideration the thickness of the layup. You lay up the part on the ouside of this pattern with plys needed for strength and then add a final finish weave layup to make it easier to prime and paint. You can cover the whole patten. You let cure for a few days. You can then eat out the foam through vent holes with acetone and it will leave the part hollow. This was the method I was going to use to re-fab a leech on my CIS car.
You can't use polyester resin. It melts polystyrene.
You can use polyester if you lay up over urethane but if it's a hollow duct you'll have to dig it out.
Keeping things light is the key. FG parts can be too heavy. That gets into Vac bagging and wicking the extra resin out with perforated palstic and blankets.
You'll admire what can be done with FG if you search a little bit on gliders.
http://www.alisport.com/ look for the jet powered glider.
Lunch hour up....