These were just something I did without measuring off to have each one be the same size, but I love them. The water in the nano is clearer than it has ever been so I will definitely be using them from now on. I used a poly clamp to clip it on the nipple of the return and even though it is visible it is not distracting because it is so small.
The 6" sewing gauge is in the photo to indicate size.
Polyester felt. It comes in sheets that are about 8"x10" but it can also be bought by the yard at most fabric stores. It is very dense and stops even the tiniest of particles. I will probably buy the stuff by the yard next time since I should be able to get it much cheaper. I just had the sheet on hand because I use a sock like this on the discharge end of my phosban reactor, but that doesn't require changing as frequently as using it for filtration does.