I know, there are already a lot of existing cases for Raspberry Pi… I just really wanted to create mine ! :)
After making this one : https://www.thingiverse.com/thing:1405805 I wanted to work on a more "slim" design. Also, I made an opening on the top to allow some ventilation, so why not make a nice one. With this model, you can choose the cat or the bunny shape, as you wish (bottom is the same for both).
Warning : this is a design for general desktop computer use, there are no opening for GPIO or camera connector !
Assembly : the opening on the front is precisely fit, so you'll have to gently push the RJ45 and USB connectors through to insert the board in the bottom part, then the rear part (where is the SD card connector will fit too. Then put the top (which should fit perfectly, if correctly printed), and close the whole thing with 4 M3x16 bolts and nuts. Then you'll be able to insert the SD card, and plug everything. :)
Update (2018 jul. 15th) : redesigned the whole thing so it's also compatible with the new 3b+ RPI board ! Also added a honeycomb-shaped top opening version, and a unicorn one, because the more the funnier ! :)