I'd recommend at least some light engineering on the hull..
Below is the build I've had for years, I don't really trade much anymore, but it served me well. FA-OFF, spin round and barrage the npc with missiles, jump out on the cooldown. NPC interdiction difficulty has changed a lot over the years. Currently, it is fairly easy to win the interdiction in a T9 as long as you also use yaw to evade.
With a relatively decent hull, even high-ranked NPC's are not a major threat. I personally never bothered with shields, they are just not good on the T9.
TYPE-9 HEAVY
Not optimized, but the hull is engineered, giving me enough time to get out of any situation.
Edit: I just noticed the engineered hull is currently missing from that T9, it is currently on my mining T9 -
T9 Miner