You should probably align the object you wish to geo merge with doors and walls, to make sure it doesn't spaz out and try to move out of the ground.
well there are a bunch of guides/ videos to help. One can be under kayaman's master index. but the main thing i try to do is suround an object with teleporters(i think they work better than boxes/walls) and they put a door on its side which is normally straighter than upside down
What I lwould do is watch all the videos like thec guy above me said and to surround the box to it can only go straight flat down that's the best way to do it, but, I stuill don't know how to wall merge with box es yet.