Valve is releasing the Steam Controller's mechanical computer aided design geometry for everyone to use under the Creative Commons license. This means the door is now open for people to add accessories and modify the look, however you still need to contact Valve for permission to sell whatever you create.
This Dropbox link contains all the eDrawings viewer files you need, including Creo Express, Native Modelling, neutral exchange and 3D print files, so you've got compatibility with a number of different design tools. If you're having model related issues, Valve invites you to contact SteamHardwareFeedback@valvesoftware.com.
The Steam Controller started shipping in October last year. It's available for $50 / £40 on Steam. We're giving the hardware and software a little more time to develop before declaring our final verdict on the controller, but we wrote about our experience so far last year.