Just like Valve did with the Steam Controller, they've gone and released the CAD files for the Index VR system under a Creative Commons license.
It's really nice to see Valve continue to do more out in the open, as releases like this will allow dedicated tinkerers to make all sorts of changes and adjustments to suit their needs. It would also allow accessory makers to easily design around it all, however if you do wish to sell anything you will need to speak to Valve first.
The release includes details on the headset, controllers and the base station. On top of that, they've included some documentation on best practices like what not to cover up, types of surfaces to avoid and more. Also included are some examples, like a "Booster" grip for the controller that could be useful for people with longer thumbs.
It will be interesting to see what kinds of things people do, like with the Steam Controller someone made an impressive Portal inspired casing although that's mostly paintwork not full-on modding, it still looks slick!
You can find the Index VR set on GitHub.