Since there are so bloody many loose carts floating around, the idea hit me to hunt for a manufacturer that produces these weird jewel boxes. But no such manufacturer could be found. Were these custom-made for NEC? That would probably explain the dropping of the design late in the Turbo's life.
My guess is that the sleeves were likewise custom-made for NEC, but these really couldn't be cut from the design...however, it seems that later ones used less plastic.
If no suitable replacement can be found, would anyone be interested in having new cases and sleeves manufactured?