Hey Kawaki,
I have had this issue before when I was testing at 7802 with an ABB robot. We found a couple of ways to get around this problem.
1. Place your fuducial grid on top of the cubes. As long as you are only performing operations on one level it will work fine.
2. Would be to change out the lens on the camera to a telecentric lens. Allows the view to be straight down. Takes the angle out of the target object.
I used #1 for my project. We did not try #2 . Figure that would work if our project went further.