Make a template (I folded a square of paper, on the point, into 4 - so I ended up with a triangle, then used a plate as a guide and cut a semi circle out of the triangle base, unfolded it and got the shape in the pic) Draw round it onto the fabric and the fleece.
Cut out adding a 1cm seam allowance.
Pin RS together and stitch approx 1cm in, leaving one of the flat ends open (which has been chopped off my pic!)
Turn RS out, fold hem in and stitch up the opening.
Get your 4 metal rings, I used keyring rings, but D rings are great too.
Find the centre of the flat 'point' and stitch a ring to each one.
Et Voila, a cosy ratty/mousey/chinchilla-y/etc-y hammock.