Is there a free QuickLook plug-in for CAD files (3D models)?
Try Graphic Converter, at least this one works for previewing the .skp files. In the free version you just need to wait around 30s to use the apps.

In some forums, there is said that to generate quicklook plugins, the developer of the apps needs to open up the source code in how their apps generate the files to display. Even maybe if someone is able to reverse engineering the apps to get the source code, there is still problems in terms of licensing. Unless otherwise the code used is under GNU license.