I've been working with a thin 3M 7847 laser marking tape, and tried various ways to hold it still while I was engraving it.
What I finally settled on was a simple jig that I made using two very thick 18" straight edges and a piece of thick acrylic.
I use a very light temporary repositionable spray adhesive on the acrylic to hold the tape in place.
I also made a simple template which I engraved onto the jig itself. This means that I can push the jig flush against the edge of the engraver to get consistent positioning of the origin each time.
I made it at Techshop!
Step 1: Material acquisition
I also used a piece of green acrylic, 24"x10", that I bought at Techshop. I used ~1/4" thick acrylic so that it would be rigid.
Step 2: Assembly
I simply used long strips of wide masking tape to attach the rules to the edges of the acrylic. If you start on the outer edges, the straight edges will match the edge of the acrylic quite accurately. I think it's <2mm error on mine.