By slip in I assume you mean not meant to be taken off during normal use.
I own this cover. The Fires slips right in and it has a cover over the screen that is held in place by a sturdy elastic strap:
http://www.amazon.com/Kindle-Lightwe...2490291&sr=8-1
Here's another:
http://www.amazon.com/Kindle-Lightwe...2490291&sr=8-1
Basically, any Folio cover will do the job.