I seam mine together with the blind hem stitch - keeping the straight part of the stitch on the right and the zigzag part catching the batting. I like how it comes out better than using the zigzag on my machine. I put the 2 pieces together, trim the edge using a ruler and rotary cutter, pin, sew, then open them up and rub my finger along the seam until it's smooth.
I've pieced several this way and they come out great. When complete, I can not tell where it is pieced.