Aluminum 15mms and 20mms tubes(diameter). The 20mms tube needs to be 1mm thick. If you have a thicker tube you could edit the freecad sources to your needs. Just print it in PLA or ABS. ABS is better(it is a good idea to use a flexible material for the internal part like ABS or Nylon), but PLA works ok. Printed at 0.1mm per layer in PLA. No supports needed. Method: Just print the two pieces. Glue the 20mms tube to the interior(smaller) 3d printed part. I use epoxi or thread locker like loctite 242. Introduce the 15mms tube inside the part. Screw the other printed part around the first one until the 15mms tube is firmly attached. You are done!! NOTE: If the plastic ring that goes inside 20mms tube breaks when you introduce the tube, don't worry. This plastic ring is only there for support while printing the part. WARNING: The pieces are orientation dependent.Do not change the piece orientation in your slicer. The screw used is specific for 3D printers and can't be printed upside down.