I gave up on fat separators long ago. I rely on "leverage".
Find a ladle or ladle-like spoon that fits into a large glass or measuring cup. If one is buying from scratch for this, buy a "jam jar" glass and one of those hanging stainless steel ladles ending in a half sphere spoon, that fits partway into the glass.
Now, use the ladle to skim the surface of the stock, by setting it just far enough into the stock for the very top film of stock to flow into the ladle. Do this all over the surface, pouring the result into the cup. The goal here is to eventually get all the fat, and some of the stock. It's ok if one ends up with half fat, half stock in the cup. That's a huge improvement on the ratio in the stock pot.
Now, let the fat rise to the top, in the cup, and this time ladle off just the fat. Be conservative; one you've removed most of the fat you can always add more stock, to get the remaining fat near the surface. Or one can just dump the stock and bits of remaining fat back into the stock pot, and do this again.
This is way easier to do that to say. Why it works is obvious once you see it in action. I can't imagine dedicating space in my kitchen to a bulky fat separator that doesn't work as well.