Since I leave the center 4-14 inch open on the bottom, (close it with the top stitching later) I can insert the 4X4 piece of plastic in from the center and move it to the side. You need to make sure you put it under the folds or you can't slide it into place. Once I have one 4X4 in one side, I slide the second 4X4 into the other side. They should just meet in the center without any overlap. If they are a little big, take out and trim. Make sure any ironing is done before you add your PLASTIC as it may distort with a hot iron?
You can use an iron on stabilizer instead of the plastic but my DIL like hers firm enough to slid into her jeans. As for adding foil to confuse the scanners? Why not, just trim your plastics a little smaller. I might make them small enough that you could top stitch without sewing through the plastic plus tinfoil as the machine may not like it.