I have has this great little Field Notes holder for a couple years now and I love it. It is the 3 x 5 size.
It has taken on a really nice worn look about it and seems to be nicer every day.
I use the larger size for my everyday notebook but I do not have a cover for them so after getting the leather area of my studio cleaned up I came up with this leather cover for myself.
It holds 3 of the smaller books so you can have a lined, squared and a blank one with me at all times.
I also added a leather tab to snap it closed so my loose papers will not fall out.