Here is my concept for a Raspberry Pi case
Since there are no mounting holes in the PCB, my thought is to hold the edges using rubber panel grommets. With ports on all sides, I decided against sides on my 'case'.
The top and bottom panels will be 3/16" thick opaque blue acrylic that I have left over from another project.
The acrylic panels are countersunk for #4-40 flat head machine screws. On top of the grommets are female threaded spacers. The fasteners from the SD card side of the board will compress the grommets and hopefully hold the board in place. Although there are no moving parts in the RasPi to worry about, the rubber grommets should also provide some shock absorbtion in the event I drop it. Dimensions are 3.00" x 4.00" x 1.21" thick.
It is modeled in Solidworks, I used the RasPi model posted by Tronetix. The last image below is a cross section view.
My idea for a case is very similar to yours, except I am planning to make the base larger to fit the 100mm Vesa mount on the back of my monitor.