This has got me stumped. I prefer carrying a smaller notebook and haven't looked deeply into the pen sheath. I think a slip would work best for me rather than a case. I carry in coat pockets, etc. .That said, the Rhodia Webnotepad looks like it might fit the bill. My journal I leave on the desk. Haven't found the perfect carry fountain pen, I love my TWSBI 580 and Conklin Duraflex -- even the Jinhao x450. But to stick in my pocket, it's usually the Kwaeco sport.
I haven't ventured into grail pen territory yet. To much to think about. I've always had a hankering for a Pelikan though. Tough to choose over the internet as I'd like to be able to pick up the pens, but the only pen store we had in town closed two years ago, and my local (Goulet) is on-line only!