To make a replacement cover for your Boppy (or other horseshoe pillow) without a zipper:
First lay your naked pillow onto your fabric. I chose flannel.
Then trace around the outside with tailor's chalk or other fabric marking tool that won't show through.
Next, lay another piece of fabric below that one, right sides together.
Then cut around your lines, adding an inch and a half (or more if your pillow is fluffier) for the width and seam allowance.
Choose where you want the pillow to stuff in. I chose a semi-flat area on one side. An area closer to the middle back may be a better idea (I struggled a bit to do the actual stuffing).
Mark this area on both pieces of fabric.
Hem it by rolling the fabric under and stitching it.
Then stitch the pieces together, leaving the hemmed part open. Stitch once with a straight stitch, then go around again with a far-apart and wide zig-zag (or serge it if you have a serger) that hangs over the cut edge a little bit. This will keep it from raveling.
I used two snaps to close the opening. If you use velcro instead, then attach it after you hem the opening (and before stitching the two pieces together).