Unfortunately, it seems the 3D-print world has a bit less of a sharing culture than the keyboard-obsessive world. There is stuff out there, but a lot of it is (a) very simple, (b) wrong or (c) hard to modify.
I don't know of the thing you're looking for, sorry. But it should sounds pretty straightforward to design - if you're willing to invest a weekend in wrapping your head around basic CAD, Youtube will teach you what you need to know for, e.g., Sketchup.