I use and love Marti Michelle's 2" fusible tape. Just make certain the heat is adjusted to the batting (on a poly setting).
I start by putting both sides of the batting together, take one side and overlap the other a few inches, then cut them in a "wave" motion no wider than the tape, remove the salvage from both sides, lay them together and lay the tape over them and iron. I only use the tape on one side.
https://www.amazon.com/Marti-Michell...-crafts&sr=1-1
The tape seems expensive at first, but it goes a long way.