As for installing a headliner, you might want to think twice about that. The C25 has limited headroom as it is, and the pop-top will make things even more confusing as you do the build. But, if you insist on putting something up there, consider long, industrial-strength velcro strips. They are fairly thin, and will allow you to easily take down (and put back up) the headliner in the event you need to find the source of a leak or do other
repairs.