In Japan, they make "pajama backpacks," for kids. They're just intended to be big enough to hold a pair of pajamas -- it looks to me like this might be one. They're often in the shape of cute characters. I have a Pikachu that I used to carry around NYC; it was just big enough for a paperback book and my gameboy.

If you look around the web, you can probably find more that would be suitable for sporran use.