1. Spread a fabric glue along an open seam and press plastic down until it bonds & dry.
2. Sew a separate fabric sleeve for the template plastic then attach this to the outer fabric layer. (the most involved but probably the most secure option)
3. This option from Purl Bee (pictured above) is the one I like best. You simply place the template plastic in between the two fabric layers during construction and then tack around each corner bringing the two layers of fabric together & keeping the plastic in place once finished.
Template plastic sheets are now available in The Haby Goddess online store.