Many of the covers we've seen are made for the regular Moleskine. This one, available on Etsy from Antjes in Munich looks thick enough to accommodate the diaries and planners.
Here's a description:
"Handcrafted/Sewed Moleskine Sleeve, made of calf leather and woolfelt. Closed with a leather band and a buckhorn button.
The colour of the felt and the size is customizable. All kinds of Moleskine and other Notebooks are possible. One Penholder ist inclued. So, it solved the problem of the missing penholder with the moleskine."