Much in the same vein as the DIY cahier cover I made last year, I made a cover for my Field Notes notebooks. The only difference about this cover is that it has two elastics and can be stuffed a little more. The leather is un-dyed and will patina over time. I quite like it and will actually use it! The one I made last year was given away as a Christmas gift because for that size, I love the original Midori Traveler's Notebook.
I use Field Notes for general lists and note taking because I don't have an insert in my standard Midori for that purpose. I think the two will work well hand in hand.
If you are interested in making one for yourself, I found this leather at Michaels and the elastic was from there as well. If you don't have a leather punch, a hammer and nail will do just fine to make the holes. YouTube is full of tutorials for whatever style/number of elastics you think you'll need.