It seems to me that everybody is trying hard to make their cases fit as snugly around a R-Pi as possible. I think that challenges with a R-Pi design is that there are cables coming out of every edge of the unit, this makes the unit "cluttered" and difficult to mount. I'm sure that there would also be instances where one would want to restrict physical access to the SD card.
One possible solution to these issues would be to make a "tub" that the R-Pi could be fitted in. This could use clips as per anhe85 :
or MarcoA :
Once the R-Pi is clipped into the unit we could then use adaptors to "point" the cables back towards the USB and Ethernet end.
At the front end we have the SD Card and power connector. The last piece of the case is sleeve that slides on from the front. The sleeve would secure the SD Card and power connector. Enclosing the power connector that could be an issue for restarts. Maybe a reset button connected to GPIO and fitted between the USB and Ethernet ports? Having a sleeve gives us more space for inserts - LCD displays, status LED to the front of the unit?
To assemble the unit one inserts a r-Pi into the tub. Then fit the SD card and power. Attach the adaptors to point cables to the rear. Fit any cables to the adaptors. Insert any USB devices and connect the Ethernet cable. Then slide the cover over everything. I'd suggest that it be held in place with four small screws through the cover into the hub. Personally I'd make the case extend out about 8-10cm at the back - this will help keep cables tidy and protect any USB devices.