Well you actually don't need multiple targets to gain now, all you need is one box. Whatever the level of your lp character, the level of the tinker that makes the box should be 10-20 points higher I think. Also, after you get high and gains slow down (I think around 95 you can't gain off boxes anymore) you can go to that ML dungeon in ilshenar with the satyr and get discorded, then sit there and lock the box, pick it, lock the box, pick it, all the way to GM.
You might want to check the treasure hunting forum for a more detailed description of how to get gains this way. =)