I'm having some problems sewing in a zipper in the back of a lycra bodysuit for part of a costume. My sewing machine doesn't have differential feed or adjustable presser foot pressure, so when I try sewing the zipper in with it the spandex gets stretched way too much, resulting in ugly rolls and bumps along the whole length of the zipper.
So far I've tried playing with the machine settings to no end, sewing upside-down (with the spandex against the feed dogs), and covering the spandex with tissue paper which I tore off once the seam was done. The zipper now is about to separate into strips I've sewn it in so many times
I *could* try serging it in based on some tutorials I found, but as that will trim away everything, I want to save that as a last resort. I also have considered pressing in a small strip of fusible interfacing (carefully so as not to melt the lycra!) to see if that will help stabilize the fabric edge so I can sew on the zipper. And ... ugh ... hand-sewing the whole zipper in.
Any suggestions as to which of those might be the best option, or something else entirely?