UPDATE - The game got a patch recently and this may no longer work
There is a glitch that allows you to duplicate the large chests.
I don't have a video at the moment but I'll do one later when I get some time.
1st thing you need to do is find a chest which will likely be close to where you land in the world so use the land scape tool to flatten the ground down to it.
Once you are beside the large chest, hold Y and select free build. Again hold Y and scroll over to vehicles , and choose a small one like a bike. Now position the bike so that it's in the middle of the chest which should have a red box around it and hold X. Keep holding X until you can move the chest. Then simply press A to place as many as you like.
You will be limited for space so I suggest going to the surface once those steps are done, you can close the free build tool and bring it back up as many times as you like to place more chests. I was getting a brick item in maybe one in 10 chests. So it will take a lot of chests open to get this I'm still around 40 off the achievement as of this moment.
I've never gone over 100 chest in the world, so I'm not sure if the game gets unstable or anything if you go over that. I usually change worlds just to be safe.
Video of glitch: sorry I didn't edit jump to : 0:43
Seems just like the gold bricks, large chests will only take you so far for this. I've 35 left to get and am getting nothing from the chests but studs now, not even from regular ones not duped. SecondOne
Reports the same in the comments. That leaves trouble makers for the rest of them.
Not sure what the best approach for getting them to spawn is. I've definitely noticed they appear often when you are in the middle of a quest or talking to a quest giver. But if any one has any other suggestions feel free to comment.