Pervert the Futter

Third time trying this mission as I had to go to work halfway through the first time and didn’t realise you can’t save. Looks like it’s going to happen again.

I’m on gate C and I’ve searched literally everywhere for the correct box but it’s nowhere to be seen. At the start of the trial I tried putting all boxes on the conveyor belt whilst I was there to make it easier and some exploded. Surely if there’s only one box for the final door it wouldn’t have exploded? I’ve searched online it says it’s in the storage room but there’s no more boxes in there.

What am I doing wrong??