I was having to put weights inside near-empty spools to ensure they didn't rock-n-roll out of position. This is a simple press-fit stabaliser, that ensures the spool doesn't wander.
The two parts snap together, and then press into the BT tray.
See https://github.com/robandwend/BoxTurtleLid/tree/main/Extras/TrayMountedSpoolKeep