I recently tried out some cheap filament on a 1Kg spool on one of our super-smooth spool holders on a Prusa MK3S+. Since these printers are direct drive, when the hot end/extruder moved, it pulled so much filament out the spool just kept on rolling and filament spilled out in a tangled mess.
Luckily our spool holders were designed with a couple of mounting holes just in case. I had no idea something like this would be needed, but as it turns out, the holes were in exactly the right place.
The design is simple: filament goes through the slot, which prevents the filament from coming off in a big tangle. So far, it's working perfectly, and I've installed them on most of our print farm. We're operating at near 100% capacity, so the reduced chances of having a 24-hour print ruined was worth the extra effort.
