I found that setting the scale to 200x200x5.20 resulted in a tighter fit. The default 200.83x200.86x5.20 was a bit too loose for my Siddament PETG spools. Other than that, it works perfectly.
The tighter tolerances are much better. I only recently discovered Siddament, it seems to be pretty good for the price, and this will make the spools AMS friendly :)
I found the ring too flimsy. Broke easily (PETG). Thicker wall would be good. And filling up the pairs of holes used to poke the filament end into, isn't helpful.