I haven't ever zig-zagged poly batting together, but that doesn't mean it can't be done.
I have used a technique from "Quilting in Sections" by Marti Mitchell that used up odd batting pieces, but if your top is already complete it's too late for that.
The technique called for basically making a 6 layer seam (maybe it was five layers? I had to refer to the book often, and now I can't remember exactly how it went together)- center section quilt top, batting, and backing, and border top, batting and backing. I thought it would be terribly bulky, but it wasn't (would have been, though, in cotton).
So I think you could seam together two pieces of poly batting just like you seam together fabrics, and open it out flat. Using a quarter inch seam allowance would probably hold it and be minimally bulky, especially if it were quilted with meander stitching or other pretty dense quilting.