Do the plans for Elite Fire Barrel Storage vary? It's all that I have left for upgrades and can't find it. I read a guide that said it was buried in the Smuggler's Den in San Juan, but I already got that treasure. I have *one* treasure map for an area that hasn't been unlocked yet (I just finished sequence 9). I haven't participated in any of the Kenway's Fleet missions...perhaps you have to get it through there.
Not that I need more Fire Barrels (rarely use them), but I'd like to have the ship fully upgraded.