I'm assuming the bag will be standing up and the tissue paper is coming out of the bag. I'd use gumpaste instead of fondant. You can shape it by laying it out on crumpled aluminum foil and allow it to dry.
Try using an egg carton covered in foil... I have the flower former that looks like an egg carton and I don't have to use the foil but it works great for this kind of stuff. I made the tissue out of gumpaste for Marge's cake and used my egg carton flower former to make all the tissue for it... works like a dream and the gumpaste dries upright.