Centerline Guide
thingiverse
Add a number 2 pencil to the hole and slide along the edges of object to make a line in the center.
Download Model from thingiverse
With this file you will be able to print Centerline Guide with your 3D printer. Click on the button and save the file on your computer to work, edit or customize your design. You can also find more 3D designs for printers on Centerline Guide.