I haven't seen this talked about much, so in case this is news to anyone, here you go.
Each G5 heavy duty 1D hull module increases all 3 hull resistances by a whopping 15% each and weighs only 1.4T. Combine those with G5 heavy duty lightweight alloy (0 tons) and you get an insane hull boost for only a few tons of weight
For example, here's a stock FDL:
And here's the same ship with 3 G5 heavy duty 1D hull modules and G5 heavy duty lightweight armor:
Effective hull integrity went from 289 explosive 338 kinetic 405 thermal to 1370/1586/1729 at a cost of only 4.2 tons, 45k credits and the crafting materials… and you still have room for the max size shield and 2 large guardian modules.
If you're short on crafting materials, putting them all at grade 3 and adding one more 1D hull module yields nearly the same results.
Having only figured this out AFTER engineering some military grade alloy and a couple of 4D hull modules, I'm off to lighten my ship by nearly 70 tons and 15%! Don't be me, commanders.
o7