This tutorial from Matt Pierce for A Continuous Lean is a winner. It walks you through the process of using stiff leather to create a stylish and protective case for small items, kits, and gear. Here, Matt designs his case for a compass and some other wayfinding goodies, but you could scale and adapt the project to fit your needs.
I especially like the use of rivets to create a belt strap on the back for attaching to your hips or pack.
A great peek into what goes into high-quality leather craft, and a perfect introduction for getting started yourself.
See the full tutorial at A Continuous Lean – Putting It All On The Table: A Wood&Faulk DIY