It's not that hard actually. Everything works in 0.5 inrements. Just stack them till there's the click icon popping up.
You can actually really screw up the quest by smashing a container over the pressure plates... The fragments seem to have individual weight contributions, screwing up the clean 0.5 increments.
Was stuck on that at least an hour I decided to just reload from an earlier save and completed it within minutes.
