I found a great piece of gear a few years ago that I have carried constantly with me since.
I think it's made by a company called Panjaro (but don't quote me on that, please): it's a field guide carrier and cover.
The cover is heavyduty cardboard covered in ripstop nylon. My National Geographic guide fits inside completely. There's a divider, then, and on the other side I can put a small (9x5?) legal pad. There's a pen/pencil holder along the spine on the inside, next to the notepad. The inside of section of the divider has a pocket where I stick whatever: used pages, brochures, map, etc.
It's held closed with a heavy velcro tab/strap (which is unfortunately a bit noisy being opened).
The spine has a nice heavy strap for carrying by hand, plus there's a long adjustable strap clipped to the top and bottom of the spine. The strap can be adjusted to carry over the shoulder, or lengthened to go over the head and across the chest (which is how I carry it, being pathetically slope-shouldered).
I've used it for several years now and just love it: between it and the shoulder harness I use for my binos, I'm quite strapped up when I'm out.
While mine is sized for a National Geographic guide (and thereby a little large for a Kaufman, Peterson or Golden), they have also come out with a larger version for those who insist on carrying a Sibley.
I find the Sibley too bulky for carrying in the field, but it's always in the car for reference, kept in an oversized waterproof paperback book cover made of ripstop nylon that zips closed.
I know you can find these covers (or you could in the past) in the catalog section at the back of the Bird Watcher's Digest. I think I paid about 30USD for mine, and it's been worth every penny. One initial purchase for the guide, then cheapo pens and pencils and cheapo legal pads as needed.
Works for me!
