Look at the possibility of using the lightest knit interfacing you can find - press a small circle on the back of the hole and use tiny stitches with silk applique thread to hand stitch it on with tiny stitches. make sure the stretch is going the same way as the garment..don't know the color or sheerness of your garment, so not sure if shadow of interfacing would show thru. Joann's carries a lightweight interfacing....I was also successful in mending a rather large hole by getting some soft chiffon in the color of the garment and stitching it onto the back in this way...used the bias in the direction of the stretch to make it "moveable".