I just got the MCM #83-14270 case, from this page: http://www.mcmelectronics.com/product/83-14270
Four snaps hold the R-Pi PCB tightly, and the two halves of the case also snap together firmly (enough so it is tricky to pry them apart: you have to pull top & bottom apart and also pry loose the catches at the same time.) It is no wider than the minimum to accommodate the board, although there is a bit of headroom inside above the board. In common with most cases, the full-size SD card sticks out way beyond the case itself, which bothered me, so I got the Adafruit uSD adaptor which ends up nearly flush with the edge of the board. However, this is a PCB + uSD socket on top, so it adds the height of the uSD below the board requiring me to mill out a slot in the case bottom to enable it to fit. If I did not have access to a mill, I would have had to cut through the case bottom completely for this. I think my result is good: when inserted, the uSD card is protected as it sits recessed about 1.5mm from the plastic lip, but it still extends enough so a fingernail can actuate the "push-push" socket mechanism to eject it, whereupon it pops out for easy finger access.
While I was at it, I added a hole for a 5.5mm +5V power jack, and also cut out a large piece to allow room for the GPIO ribbon cable (no slot is provided in the case for this). After installing the 5V jack, I realized there was about 2mm of interference with the 220 uF capacitor at the power input, when the case was assembled. So I moved the cap and rewired it, but probably I should have put the 5V jack farther over. This was with the Gen 1 board, I don't know if that cap moved on Gen 2, but I assume not.