I know you want to build it but take a look at Robern. They make one and it could also give you ideas for having one made.

I have seen this before... but it was for storing shot guns!

So you would be inserting shelves between two existing studs. Then a framed mirror was mounted on the wall.

You could even buy a jewelry storage mirror and mount it into the wall. I have thought about doing this in my own closet. I got one off craigslist, for $30. http://www.qvc.com/scripts/detail.dll?item=H86082&cookie=set

I didn't find my original post, however I found a really neat link so the pages of a 1925 popular mech. mag. I couldn't help but share!

http://books.google.com/books?id=29oDAAAAMBAJ&lpg=PA1018&ots=xnpEYBPHbc&dq=dressing%20mirror%20between%20studs&pg=PA1018#v=onepage&q&f=false

A lot of my neighbors did this in their tiny kitchens to make pantries. Basically just lined the opening with a sheet of formica or whatever, then put in the brackets and shelves, then in their case a bifold door. Shallow can hold a lot:

{{gwi:1443704}}

I've always liked this one on houzz.com...

https://www.houzz.com/photos/olson-design-and-construction-contemporary-bathroom-san-francisco-phvw-vp~14578-Design---Construction-contemporary-bathroom-san-francisco

[contemporary bathroom design[(https://www.houzz.com/photos/contemporary-bathroom-ideas-phbr1-bp~t_712~s_2103) by san francisco interior designer Olson Design and Construction