Slug, that makes perfect sense.
I think one way to go about it is to shape your entire tank sans tunnel, then wrap the glass so it goes like a couple inches or so under the tank. Maybe stick some hooks in the foam and let dry hanging upside down. When you make the tunnel piece, just wrap the concave. The spandex should do well. It's convex curves that would be a pain in the ass.
When both pieces are dry and cleaned out, cut the excess fiber glass from the under side of the tank so that the tunnel fits snug, glass both pieces together, let dry and finish up with a tank sealer.
I haven't tried a tank as it seems daunting, but I'm trying to visualize the process.
How do the pros do it? Negative molds or blow molding?