The download below contains the .SVG design for a Raspberry Pi 1 model B+ acrylic case.
Here are some pictures of the case:
The next download contains the .SVG design for a Raspberry Pi 2 model B acrylic case.
Thanks to smartroad, you can also find design files for 3D printable cases at http://www.thingiverse.com/thing:1077472.
Thanks to NickRBrewer, individual 3D-printable parts for a whole RetroPie arcade cabinet can be found at Thingiverse, e.g., at http://www.thingiverse.com/thing:1016176.