Piece A, the exterior part, has to get a thick batting, the Pellon or G700 is not thick enough.
You can take thin polyester, but I prefer the standard cotton batting. I love the touch of quilted cotton! It's quite thick on the bag, so there is a lot of protection J.

If the fabric is not directional, you can take one piece of fabric.

If it is directional (like mine, all the guitars go in one direction), you should cut and sew the piece in the middle.
It doesn't have to be the exact middle, 1 inch doesn't matter. But 2 inches might be too much, the seam might show on the side.

I cut fabric and batting about 2 inches larger than the pattern says.

The reason:
I quilt piece A and the batting together, the finished piece always shrink during the quilting. Ok, it doesn't shrink 2 inches  - but I want to be safe.
When I made the first bag, it ended too small – I had to start all over again with a new piece A and batting. This won't happen a second time!

I use Nylon and Superior Bottom Line for the quilting, but of course you can use whatever you love.