The side parts of this type of sporran is thin flexible leather. Yes - Just air inside, unless you want to add some personal pockets or attachments when you make it.

Before you cut the leather, make several mock-up sporrans. Try out different sizes. Make one in thick canvas or better before you cut the expensive leather.

Look at many web-sites to get ideas about attachments or pockets to put inside, because these should be put inside before the outside is sewn up.