[SteveW]

I too have built a few model aircraft using this method. Use Blue foam as it cuts and sands very well. Once you have the shape you need cover the foam with parcel tape. Then give it a coat of spray mount to help stick the glass to it. build up layers of glass with your chosen resin (Epoxy does not attack foam). When finished sand fill and paint. Next make a hole through to the foam and pour in thinners. This is very very messy as it desolves the foam. when all the foam is out you should be able to pull out the parcel tape. If weight is not an issue just leave the foam in place.

In the summer I will be making a roof spoiler like this.