I use the paper cutter to slice through the welded seams on the back. It's a messy affair, but I actually make some progress.

At the bottom, near the ball joint, the seams are easier to slice, and eventually, the knife breaks through the surface even on one of the straight edges.

I thought I might be able to cut the case open now, but in some places, the knife simply won't get through. After more than half an hour's slicing, I give up.