As you might know I offer hundreds of millions of in game money from time to time to raid my base. Not once has any succeeded and this is due to some simple things
Location is very important. If you build in small locations you are doing it wrong.
Some good places to build are left and right rooms in front of waterfall in sewer, industrial first right sided building, warehouse, behind 3 buildings left of beach in that alley, and island base. An amazing design are zigzags my personal favorite and most effective.
What's a zigzag? Well a plethora of fences where you make the player move left then turn the corner then right then turn a corner. These sharp turns are forced by bangable fences which you can shoot through. I suggest you and a friend go to a place of the mentioned spots and build a base together. What I mean is you build a base and then let them use their 100 props to addon yo your base.
Now you have a unraidable base! Yw