I designed and created this simple funnel for sliding filament into a single PTFE tube. This is especially useful when using the external spool as a supplement to the AMS filaments, e.g. printing with TPU 95A (which are not AMS compatible).

A standard PTFE tubes slots in perfectly. When used with a Bambu 4-way, or a community designed PTFE Y splitter, guiding filament into a PTFE tube is easy with just one hand.