I had a similar situation and I had the seam going vertical to the roller bar--I made the seam a little bigger--maybe 1/2" and pressed very flat and since only had about 3 rolls to the quilt (lap quilt) it worked ok. It might work to have the stretch attached to the roller bars if you don't keep the backing very taunt. I have 4 lap quilts for Christmas, all with a minky type backing, that I had planned on having to piece the minky for the width and had bought enough to allow me to cut it in blocks and piece a strip to make sure to have the seam horizontal. Somehow I got lucky and all were wider than the 59" the clerk told me--except the one that was a narrower quilt! Good luck!