I love to sew as well, and guinea pigs are such a good excuse to buy fleece.
I have made cage pads anywhere from 12” square to 18” x 33”. I put fleece on the top and cotton broadcloth fabric on the bottom. The middle layer can be anything lightweight and absorbent, like a terry cloth kitchen towel or a beach towel cut to size.
I have used a server to just layer everything right side out and serge around the edges. And I have also turned everything wrong side together and stitched the edges, leaving a gap to turn it right side out.
The larger size is nice because each one covers a little more than a third of a C&C 2x3 cage. So 3 will cover a whole cage.
The fleece side is comfy for sleeping, the plain cotton side is great for a hay pile because hay and hair don’t stick.
I prefer the anti-pill plush fleece because it is a little thicker and seems to be easier to clean.
I have also repurposed quilted waterproof mattress covers to use for cage liners under a layer of fleece. I’ve gotten some nice ones at thrift store, and just cut them down and finished the edges.