Not to take anything away from the solutions above, but here's another option that some may find easier, quicker or just to break the monotony of this achievement....
All credit for this has to go to BTHRZeroX on TrueTrophies. I'm merely cross-posting it here to give it more visibility.
I found that by adding a fourth safety deposit box across the face of the three shown in the video I could generate 21m every 10 minutes. This is with the positioning of your character below the safes in the hole being spot on (largely down to luck as once you're in the "copy" tool the game moves you as it sees fit) and the correct rhythm of pressing "A". I found once every other second worked best. Too quickly seems to upset the game.
I also tried with a lot more safety deposit boxes, but no other combination worked as effectively as four. More than this and you seem to a) not have enough radius to pick them all up, b) get thrown out of the hole a lot, wasting loads of time and c) crash the game much more often.
It still took me 10ish hours of mind numbing grind, but I got there.
As mentioned elsewhere, save OFTEN. I did so every 10m and still lost quite a bit of progress to crashes.