I hate the sandwiching part, too! However, here's what works for me - Elmer's School Glue!!! Someone on the QB suggested using it for basting; I tried it and may never go back to thread! I've only used this method, with Warm & White, so don't know whether or not it'd work with poly batting.
First, lay out your batting, then cover with backing. Smooth but don't stretch anything. Now fold back half of the backing and drizzle fine lines of glue over the exposed batting. Try to not "glop" it on but use lots of skinny lines, swirls, whatever. Carefully fold the fabric back over the batting and smooth. Pat it down, gently.
Now, if you're in a hurry, heat your iron and gently press the glued section, all over. Don't drag the iron, just touch to fabric for a few seconds, then lift. If you are not in a hurry, then you can just go on to the other side and let the whole thing air-dry.
Fold back the other half and do the same.
Once the bottom has dried/set, flip the entire thing over and do the same with the top.
The needle has no trouble going through the glue and nothing slips! I also use a dual-feed foot, to do the quilting, which also helps to keep it smooth.