I would just use the Reaper's flag, but it's context in regards to how you intend to use it is important, as is how other crews perceive it.
For example, I waited for over 3 hours at Cannon Cove, and then at Shipwreck Bay with it flying and while standing at the highest points of both islands and nobody ever came. All I wanted was for someone to sit on their respective bone thrones with me - but there was no way for anyone to know that. I was even carrying a boatload of loot ready to pay them as a reward for their help. Still, nobody came.
The entire time, a Reaper's Chest had not been far off. I retrieved it in the Hope's that it would get somebody's attention. Placed it directly on the thrones too, with the hope that someone would take a close look at their map table and understand the importance of its placement. Still no dice.
I decided that the flag was keeping pirates away, so I took it down, and waited a bit longer. Nothing. Not a sign of anyone anywhere.
I was starting to think that the server might have been empty.
So, I decided to sail towards the RBH. As luck would have it, as I did so, a ship sailed right up to SWB. I immediately anchor-turned and pulled out my speaking trumpet. It took 3 tries during the chase for them to finally decide I wasn't a danger to them. He helped me as I helped him, he got paid handsomely, and I went to go eat dinner with my family.