Offline raidings are actually a sort of tower defense mode were you play on the attacking side. Thats not PvP. Pvp requires players in oposing forces fighting each other. Since any defense will always be beaten with the use of the proper force, going the turrets/traps or npc route is pointless.
I suggest an offline block to render bases invulnerable 100%, like claim blocks did many alphas ago. If you want raiding the base owner should at least be online, or if its a faction then any member be online.
Any issue with possible exploits or abandoned bases can be handled by server admins.