What can I use to fill a tooth space in my Invisalign tray? (Photo)

My 10th tray and from now on has a space for my left lateral incisor. Eventually there will be a space for the right lat. incisor as well. My treatment doesn't come with a pontic tooth although there is space for one in the tray. Now the space just fills with saliva. My ortho. suggested perfect-a-smile aligner coloring, but its more than i'd like to spend. He said some patients use nail polish, but I don't think that is safe to have in my mouth for 22hrs a day. Any other cheap but safe options?