I needed a quick way to splice 4mm OD  PTFE from my Bambulab AMS to my P1P, and needed it to have no resistance as it was being fed through. The center guide is slightly larger than the 2mm ID as the filament exits the AMS side, and narrows to exit slightly smaller than the extension tube. This helps the filament smoothly transition from one tube to the next without hitting anything along the way. 

1/25/23 v2: redesigned the cutout for a smoother transition, added a bit of a lip on the openings to help hold the tube in.

1/26/23: Putting it back in Beta testing. It works great with my PETG, but when I try to use my Bambu Matte PLA it pushes the connector apart and feeds the filament out in the air. I will appreciate any comments and remixes that help solve the problem.