I usually just butt up the edges and use a big zig zag and it works just fine. Maybe it's the type of batting you use. You could have the batting pieces together like you were going to sew them, overlap a couple of inches, cut a very gentle curve through the overlap. Remove the overlap. You should have a matching curve edge. Try zig zagging that together. That should help you know where you need to match up as you go along and hopefully avoid stretching. HOpe this is clear.