We built a box but it's barely too big for our mini guernseys, so it'll be even bigger on our nigies. We put towels on the bottom to make them taller but they could just squeeze their heads through the neck hole. We ended up keeping them in the box but I had my hand on top of their neck, forcing then to keep their heads out. I've read where people would put towels in the neck hole to make it smaller. I'm not really sure if this makes sense, but I hope it helps if you can't get/make a smaller box.
it's a bit big for some of our kids so we use rolled up towels to adjust as needed. I'd rather they be a little cramped vs dangling. You also don't want your holder to press down on the head as you don't want to cut off blood supply from the carotids. When we first started disbudding I thought we'd killed one of our bucklings, he was almost limp coming out of the box. Ended up being fine but was scary. After that we started using the towels and haven't had an issue since.