After several prototypes I finally came up with a really good solution for covering the Koralia Nano powerhead. I've documented the steps with lots of pictures in case anyone wants to create one for themselves. All you need is #7 needlepoint plastic canvas, a whole bunch of small zip ties, and the Fluval U3 filter foam.
First cut a rectangle out of the canvas matching the size shown in the picture.
Insert zip ties as shown. Zip them up and trim them.
Do almost the same to the other side of the canvas rectangle but spacing the zip ties a little differently.
Fold the rectangle to form a cylinder, overlapping by three holes, and add zip ties.
Cut a square piece and then cut it down to a circle.
Cut another square, cut it down to a circle, and cut out an opening as shown.
Lay the two circles 45 degrees rotated from each other and join them with zip ties.
Thanks,
Harry