Valve has released the mechanical CAD geometry for the Steam Controller under a Creative Common license. Files covering all the joypad's externally visible parts are provided, allowing users to edit, create and share new controller designs as they wish. However the license doesn't allow selling of such designs, if you think your designs are marketable and people will pay for them you will have to contact Valve to negotiate.
In a Steam Community post, Valve said that a variety of files are in the downloadable archive it has made available. There are files for software tools ranging from Creo Express and native Modeling, to neutral exchange and 3D print files. This will give most users access to the 3D data they need to work with the design tools they currently use.
Thoughtfully Valve has started the modding initiative rolling by sharing some alternative 3D printable designs for the Steam Controller battery door. A couple of choices of design provide controller users with a choice of a recessed USB adaptor socket or a totally enclosed USB adaptor storage slot (as pictured below).
Of course there's lots of other modding opportunities available to end users now. You would make the controller perfectly ergonomic - for you - using your hand grip prints, perhaps. Alternatively, if you are interested in old arcade games, then a button layout customised for SNK games, for example, might be a great project.
The main 3D geometry files zip archive file weighs in at 800MB and is downloadable from Dropbox. The two example battery door designs are just 50MB each.
