This is technically a bug, but since it has no game impact, figured I'd put it here.
When you design a magazine, and you check off the "Show Next Tech" option to let you build a future prototype, Structural Shell shows up as an option for your armour type. It's by far the best option, with a very small size and a very low cost.
Fortunately, any (FP) module of this sort that you design can never be researched, so you can never actually add them to a real ship.