Materials

• Plexiglass tube. 40 mm inner diameter, 50 mm outer diameter.
• Metal pin. I have used a bicycle spoke.

Procedure

1. Cut the tube pieces as shown.Make the cutout slightly smaller than needed

so you can adjust with a file. 
2. Drill holes for the pin so that it will be locked in place when the tube pieces are assembled.
3. Make an extra cutout on top of the small piece for the rotating wheel

of the ball entry mechanism. 
4. Fasten the pieces together with adhesive tape. 

Glue them together when the rest of the clock is finished and everything is working.