BMatthews

If you make the blue or pink foam parts the right size and shape you can then make up a divot cutting tool that makes spot holes of the correct depth over the surface. Then use those to sand the rest down to match and you've just allowed for the layup going over the foam. With any care at all you'll easily be within 1/32 of an inch to the so called "true" shape. And given your self proclaimed level of fussyness I'll bet it would be closer than 1/32. Doing the parts that way would certainly be far less time consumning than making a plug, mold and finally laying up the parts you need. And on a 1/5 model I'd suggest that this is close enough for anyone that doesn't walk around with a dial caliper in their back pocket.

Weight? I wouldn't be worried that much about the few extra grams that a glass over foam and then melt away foam part would have compared to a regular femail molded item. Besides there's the old method of pushing a regular balloon over the part and letting the air out so that it ends up with a compression elastic over the entire part. That'll squeeze out a lot of the excess resin and compress the layup so it's evenly thin overall. Did a glass over balsa nose cowl this way some years back. The results were more than I was hoping for. All I did was sand off some bits of the latex before painting. If the bit of balloon hadn't stuck it would have been easier than that.

Give the method a try on something for an existing plane. Then if it doesn't work to your satisfaction you've lost nothing but a bit of time and materials and a 10 cent balloon.