A small, very modest, often out-of-the-way place.
The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company.
http://www.thefreedictionary.com/hole in the wall

simple, petit


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Note added at 13 minutes (2008-02-06 15:14:23 GMT)
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sans prétention

A small unpretentious out-of-the-way place
http://www.wordwebonline.com/en/HOLEINTHEWALL

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Note added at 19 minutes (2008-02-06 15:20:03 GMT)
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The Online Etymology Dictionary explains that the slang term for "small and unpretentious place" was first used circa 1822. In the early 1600s, "hole" meant "a small, dingy lodging or abode." Perhaps back then, "unpretentious" implied that nobody had cleaned for a few days.

Peak English concurs on when the term was first observed, as well as the definition -- "a small, simple place." So, our hole in the wall might be a bar, and someone else's could be a sandwich shop or ice cream parlor.
http://ask.yahoo.com/20060601.html

hole in the wall, a small or confining place, esp. one that is dingy, shabby, or out-of-the-way
http://dictionary.reference.com/search?q=hole

manifestement, les références sur la Toile ne manquent pas...

Pourquoi pas, selon le ton du texte ...

A hole-in-the-wall is a rundown place, usually refers to a bar favored by locals. 'boui-boui' is a little bit slangy, but I think it works in your context.