I studied one close up and realized it was simply a zipped bag with one of the raw edges sewn differently.
I made the bag the same way for a regular zipped bag, sewing the edge closed with the zipper tab on the edge.
For the opposite side, I made a little tab with the stripey lining fabric. I opened the zipper and centred the zipper and sewed that side closed aligning the raw ends of the folded tab between the two layers about 2.5cm from the zipper. I then zigzagged both raw edges.
I turned it right side out. Although it turned out very well, my hedgehogs are upside down! Next time I need to remember that the open end of the zipper is the top.