My version of the extension clips to keep the filament spool in place. These screw onto the ends and include a printable washers to hold everything in place firmly. The body is 60mm long, and extend the ends of the spool by 45mm. They are working great for me.

Print six copies. Use an 25MM long M4 or M5 button head screw to hold in place. a 1" 6-32 should also work fine. Install the screw from the inside out to keep the filament from catching on the nut or the end of the screw. Note in the photo I am using a countersunk screw because it is what I had, but for best results I would recommend a button head screw to reduce the possibility of the filament catching (though I have never had problems using these screws).