twentyfourteen : make slider default option for featured posts

Hi everyone. I rarely post but I can’t seem to figure out how to do this.

I would like to make the twentyfourteen slider the default layout for my featured posts (instead of the default grid layout)

I am using child themes of twentyfourteen on a multisite network (with no active public registration : I create each blog after users fill out a form).
However, whenever a user switches from another one of my available themes to a twentyfourteen child theme, I have to switch the layout from grid to slider, by hand, after the theme switch. (The customization options are not network activated for blog admins