Glass vs. build up
Make your mold out of pink or blue foam and sand it smooth. Then try glassing scrap pieces of foam using 1.8 or 3.6oz/SQ Yd E or S glass cloth,experiment with both using small pieces. Once you decide on the cloth, you want to use you could do one of two things . 1. cut the mold in half and then coat it with expoy resin and then mold release to made two matching halfs and then be able to make more fuse's. 2. coat the mold with a light coat of expoy resin then lay some glass and add another coat once it dries and add more till it feels strong enough, then just use acetone to melt the foam inside your fuselage, you end up with a hollow fuse.
Hope this helps, you may want to look at Ezone for more tips in the foam section under Electrics.
Mike