This is a modification to Roberts feeder Guide, so initial credit to Robert for his design. I use a 13mm diameter bearing with a collar guide on one side and a hobbed gear on the other. This modified guide has very little clearance to prevent the flexible filament from escaping and a hole in the back that allows you to insert a pipe cleaner soaked in oil to lubricate the filament as it begins to enter up into the Bowden tube and helps reduce the friction. depending on your set up and length of screws sticking out of the feeder gear you may need to fettle things a little.