The Hera CQR CA Compliant Buttstock is a nice option for your featureless AR15 build. Light weight and attractive, just remove your existing pistol grip and install it over your mil-spec sized carbine receiver extension (note: mil-spec size is recommended, but we were able to loosen up the screws and get a commercial tube to work). Be sure to also remove any other evil features from the rifle and you have a fully compliant, featureless rifle that can utilize detachable magazines and does not require Assault Weapon registration in California.
Moving out of California in the future? Simply remove the insert and revert back to a traditional thumbhole stock.