The boxes I had bought at IKEA along with those stuffed animals to work on object matching with presented another challenge to us.
Not with using the boxes, but just approaching them. He avoided the boxes and was very reluctant to come up to them. He got so scared, he started to shaking again, just like he used to.
Things like this are perfect for an all-positive approach. For any interaction with the box, he got a click and a reward, this time a piece of deli ham. I wanted to give him something that he likely finds more enticing than even his beloved bread balls.
This got him at least looking at the box and stretching his next out towards it. I approached this as a shaping exercise, so no cues from me, just clicks and ham or nothing. I wanted him to approach or give attention to the box on his own. I didn't want him to feel like I was pressuring him or to add any more anxiety to what he was already feeling.
Working on this took a lot of energy out of him, something I remember from working on fear issues before. He needed a break, so I took him out for a leisurely walk (more sniffing and marking than walking) and then he took a nap when we got back in and settled (and cooled off).
This is something we'll have to keep working with so he can get over this as well.