This one's been awhile coming and it's the cheif reason why there haven't been more posts for the past couple weeks. A brother-in-law requested a messenger bag he could roll up and stow while riding his motorcycle and after a couple months to ponder the design, a few proof of concepts, and a lot of drafting I was able to come up with this.
For all that, the design is fairly simple and fundamental, which should also make it reliable. The front is made from a flexible, almost fabric-like utility leather and the back uses a more rigid bison leather (though normally I expect I'd use stoned oil since it's half the price and I can get more of it). In either "mode" the lid fastens closed by pulling a tongue through a keeper. It's faster than a buckle and I find it more convenient and reliable than a snap for something like this. The strap was made from some light veg-tan so it'd be bendy enough not to get in the way. I used a polyester lining because it has a plastic interface that works really well with contact cement. However, I'm not really happy with how it worked out for the edging. It's functional but I think a different material would probably look nicer, especially with the double seams.
All in all, it's lightweight (about .76 kg with the strap), super sturdy, and easy and quick to manage. The biggest drawback is the ~50-60 hour build time. Hopefully I can pare that down without modifying the design because I definitely intend to add this as a regular item on the store.