This is a good all purpose set of instructions to make any size cage hammock for cat, kitten or ferret. It is non specific because it is intended for recycled materials. Reuse whatever appropriate materials you have at hand. But the design is basically the same.
Hammocks can be made with clips attached or only tabs for detachable clips. Metal clips can chip the enamel inside a washer or dryer. I find I need several, one is always in the wash.

For these projects I used
Polyester fleece stadium blankets from the thrift store for $2.00 each
Polypropylene webbing from a belt strap cut into 4" pieces.
and 4 swivel clips from an old purse (they don't HAVE to match)
cheap shower curtain hooks

Step 1: Make a Template or Paper Pattern

If you have one you like, trace it on paper and cut it out.
Or you draw something freehand.
Or cut a template from cardboard, this is best for rectangular ones.
Keep reusing the cardboard template to trace new ones.

For this particular design I made half the pattern, so that I can cut the material folded.

These are both 15 inches long by 13 inches wide, though they can be made wider or even smaller to fit smaller cages. I find with this size i can actually get TWO into one cage on different levels.

If you are making the 'sling' style, be sure to make the corners taper to no less than 4 inches wide to allow for webbing AND the turn.

Step 2: Cut Out Material

Pin the pattern to the folded material.
Trace and cut out TWO sides.
leaving 1/2 to 3/4 inches for the seam allowance.

If the top and bottom don't match perfectly, they can be trimmed AFTER you sew,
but it won't matter because it will be inside the hammock.

Step 3: Sew Around Edge While Attaching Tabs

Start half way down one side and sew until you get within a few inches of the next corner.

insert the webbing tabs with or without swivel clips into the corners pointed inwards.

You are sewing around them and through the two layers of webbing.
Take 2 or 3 passes over the nylon webbing.

STOP sewing when you get within 4 inches of where you started.
You need this gap to turn it inside out.

Cats and small animals are not particular about fashion.
if the sides don't match perfectly don't fret.
The goal is to create something that will survive use and washing machine cycles.

Step 4: Turn the Hammock Inside Out

When you have finished sewing all the way around except for 4 inches in the middle of one side.
Pull the tabs and clips through this gap in the seam edge.
then finish by hand sewing the seam closed.
Use an invisible seam stitch if you can, otherwise just a secure overhand will do.