So I got the reflectix also and cut out the windows. Is the duct tape to keep the reflectix from fraying or are you also adding some material to give the reflectix a more stiff back like cardboard? I can't tell.
I covered the one side with black tape just so it wouldn't be noticeable that there was anything in the windows from a distance, but the heat causes the duct tape to shrink the reflectex (not to mention they get so hot you could burn your fingers removing them) so I made new ones and only lined the edges (in orange) to keep the stuff from fraying or coming apart, though it seems mostly unnecessary to do so. Also, because they shrink in the heat with tape I cut them a lil larger and after a couple days of use they now fit perfect.