Although I sewed this by machine, it is simple enough that it can be done by hand. I do recommend reinforcing the sides of the opening as this is the most likely place for your stitching to break.
Like this pattern and want to try something bigger?
Step 1: What You Need:
Polar fleece, 9" of 60" wide ( 23 cm of 150 cm wide)
2 sew-on snaps
Either a needle or a sewing machine
Step 2: Draft the Front Piece, Aka: the Master Block:
Draw a line 1” (2.54 cm) away from the original water bottle tracing, going all the way around. This 1” includes ½” (1.2 cm) of seam allowance and ½” of ease. This is the master block and the front panel. Cut 1 in polar fleece.
Step 3: Draft the Back Pieces:
To the Bottom Back piece, tape a piece of paper to the cut edge and add 1½” (3.8 cm) hem allowance. Cut 1 in fleece.
To the Top Back piece, tape a piece of paper to the cut edge and add 2¼” (5.7 cm) hem and overlap allowance. Cut 1 in fleece.
Step 4: Assemble the Back:
Measure across the hem, and sew snaps at 1/3 of the distance from the side and centered in the hem. If you measured correctly, the top and bottom pieces should line up and the overall shape should match the Front piece.