This instructable will guide you through drawing a part in Autodesk 123D, having it 3D printed, making a mold of the part, and casting the part in polyurethane rubber. I will gloss over some of the mold-making and casting since there are already so many good youtube videos on the subject.
The MuscleTreads™ water bottle massagers are polyurethane rubber rings made from Smooth On's Vytaflex® 60. They are designed to fit over a 1-liter SIGG bottle (or any bottle ~3.2" in diameter) to turn your water bottle into a back massager (used on the floor or against a wall or chair).
The MuscleTreadsTM water bottle massagers are patent pending. You are welcome to make these for non-commercial use.
(This instructable has been updated to include instructions for how to make a one-part, open mold for this project. See Step 13.)
Step 1: Start drawing the part in Autodesk 123D
Open a new project/file, set the units to inches (labeled "in Eng"), and scale to 0.1in.
Select the front view from the view cube, start a new sketch (the pencil icon in the toolbar), and select the XZ plane.
Select the circle tool and draw a circle with center at the origin (0",0") and radius 2.5"
Draw another circle with center at the origin again and radius 1.425" (a shortcut for this is to right click and select "repeat" -- this will put you in the circle tool again).