The shower knob in our guest bathroom was spinning freely, so I needed to do something fast.

I quickly came up with a design that will work using 123D software (free). The design and print files are available at:
http://www.thingiverse.com/thing:102624

Step 1:

Life is not perfect. The hole in my first knob was too small.

Step 2:

Rather than redo my 30 minute design, I just "enlarged" the print to 119%.

Step 3:

The hole worked, but I had left too much "shoulder" inside and the screw wouldn't fit. I did a two minute design tweak and printed again.

Step 4:

Put the knob on and screw it in.

Step 5:

Because my knob is long and does not contain a "stop" to limit its travel, I wrote the word "off" at the top shutoff position.

Beauty and a "stop" are desirable upgrades for the knob, but this is functional, costs about one dollar, and is more socially acceptable than a pair of vice grip pliers.

As a follow up, I found a way to repair the old knob:
https://www.instructables.com/id/3D-Printed-Repair-for-Broken-Faucet-Handle/

Introduction: Printed Shower Knob

Step 1:

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Step 4:

Step 5: