Hi, Saintone here. Seems to work this way, hoping for a second Practonium. I managed to get the box just now.
You need to do the questline enough to unlock the last two NPCs on the map (18 total, 16 initially before the quest). This requires you to be a minimum of level 317.
Then talk to all of the NPCs, making sure to activate both of their dialogues (they each have two things to say, and it’s RNG). A quest noise is activated on each dialogue. Take your time and activate both dialogues.
Curtis will have a special quest after (he has dialogues too), called Forgotten Refugees.
The quest is to go to Knight Commander Uska in Klaipeda, who will give you a special box consumable. Using the box gives you a chance of two kinds of cubes, both with the same name and icon, along with either Lv. 4 Keistas Recovery Potion or Mithril Ore.
In one kind of cube, you get the special consumable box again. In another, you get the Practonium. If you get the Practonium, you no longer get the box. I haven’t gotten it yet from this method but a friend says you cannot get the box again after, so this is an RNG surefire and easy way to get one per character.
This method seems much easier than trying for the material from the 315 cube, since there is a LOT of junk in the cube and it’s a somewhat difficult dungeon. There is also no confirmation of the material coming from 330 world bosses, however it’s not impossible I guess. They are very seldom done.