There are no civilian shield generators, which makes them a non-issue for raiding fleets. The only ships that can carry shields are warships and those should not be the target of pirates. Are ship designs of the pirate faction known by now? Are they using something like WW2 submarines, which are small, cheap, stealthy and numerous or do they use capital ships as raiders, which can slug it out with a small escort?
My last raiding design was a small fighter with a single shot 12 cm railgun, minimalistic power supply, short range active sensors, short range BFC and a speed that is good for hunting civilians and not running from warships. In combination with passive sensors like tracking stations or sensor ships, they could cause a lot of damage. The small size and acceptable speed allowed me to pick targets and evade fleets pretty well. The cost was significantly less than 100 BP, which makes them expendable. I bet that you can hardly defend your convoys economically against such a thread.