I have a Mission Workshop bag that has a lot of cargo space. The only issue was that my field notes notebooks kept getting wrinkled from other items in the bag. So I decided it needed a little protection.
There are two sleeve types. One holds two notebooks snugly. The other has the proper FIELD NOTES type with the official flask moniker, it is built to hold my favorite pen from MUJI hex pen .3mm
Picture also contains my other favorite notebook from WORD

Printed on a MakerBot Replicator with 0% infill and 3 shells for strength