Just putting this up quickly but may need some updating. Inspired by http://www.thingiverse.com/thing:223412 I wanted to try creating my own in OpenSCAD. I've tried to make it parametric but it needs some comments, so I'll update it.
You can change the motor mounting details and distance to the rim etc. I wanted to use it to quickly be able to slim down or beef up as needed after some flight testing to see how light it could get and still protect the quad.
This is a work in progress :)
Update: Added quick photo
Update: A much needed refactor to simplify the code and remove duplication and redundant sections. This also has slightly smaller tabs as I was reducing weight.
Update: Added just a single corner stl. I've not tried it, but you could print 4 corners then super glue them together. It has rotational symmetry so all the corners are the same. You could use a bit of tape for strength.