You might consider leaving the shell hardened and just closing the back, then doing something with the shoulders and head, turning it around of course, to make the flap for the sporran. Many of the leather sporrans out there are fairly rigid---the shel already provides that rigidity. If you need more space inside than the shell alone allows you could put a fairly stiff leather back on it to spread the shell out into a larger capacity. I think it would look cool as heck (I used to live in FL and had these things all over the place---armored rats is what we called them---and would like to see one put to good use for a change). I don't think that they had armadillos in the highlands so it falls out of the traditional category of sporans, but I still think it would look cool as all get out. Pictures of the final product are a must.

Sorry, but have to inject this armadillo joke:

Why did the chicken cross the road? To prove to the armadillo that it indeed COULD be done safely.