I bought a BookArmor case (fits most 6″ readers) from Amazon for about £25. It leaves enolugh space to allow you to plug in the USB lead without removing it from the case.
HOWEVER: this case uses a “click lock” adhesive strip, two of which must be attached to the back of the reader. These mate with the lining inside the book cover in a manner similar to Velcro. In order to lock the device in place you have to apply a bit of pressure on it. The first time I did this the reader no longer came up as a removeable drive and when I turned it on it wouldn’t respond except to tell me that I’d run out of available memory. Turned out that in installing the device into the cover I’d flexed it too much and dislodged the compact flash card.
In short, the case is sturdy and will protect your eReader, however the 511EB is not particularly robust so you really have to be careful when installing it!