There are some reports of problems with the Kobo Aura One’s official cover - specifically, some covers fail to put the device to sleep when closed. Kobo decided to put the power button on the back, so it is difficult to find a universal case that is compatible with the device. However, some iPad Mini cases work with the Aura One. The right case to choose needs to use elastic bands or flexible bands that hold the device (most iPad Mini cases come with a form-fitting hard plastic casing. Second, a large hole at the back for the camera is important so that there is easy access to the power button.
As an example, I’ve found Belkin’s ‘Smooth Bi-Fold Case for iPad Mini’ to be compatible with the Kobo Aura One.