So this is something I have made a guide for, which is available at the Completionist discord. This is one of those things (like the gifts commendations) that was not at all thought through when changes were made. If it was thought through, that's embarrassing. With the removal of these types of quests from barrels, and the removal of gunpowder barrels from the Merchant voyages, they have become much more rare to obtain.
The best way, or rather the quickest way, to obtain this now is through a dedicated grind. Below I will post some images for different islands. You gather 2 other people, and all 3 of you set sail on your own individual brig.
Then you put down a quest, where you dive to a basic Gold Hoarder voyage. You keep putting those down until you get one of the islands in the images below.
Dive to the island, follow the guide, dig up the Gold Hoarder chest (just dig it up once so the voyage completes, you're not selling it anyway), and put down a new dive voyage to one of the other islands.
Depending on what all 3 of you need, you invite eachother to eachother's brig once you have something one of you needs. This can be a voyage for a gunpowder barrel, a wood crate, fruit crate, or cannonball crate.
Follow the route in the images, look to see if any of the journals spawn, and look for bottles on the beaches along the way.
It is extremely tedious, but there's no other way to do this with any sort of replicable results.
Crook's Hollow - https://imgur.com/oJYcTb8
Lone Cove - https://imgur.com/WpGWRlh
Mermaid's Hideaway - https://imgur.com/2PSVH7x
Snake Island - https://imgur.com/lY2Zy92
The Sunken Grove - https://imgur.com/nZpT8xI