I corrected the dimensions of the STL files provided by andymark for a 3D printed string potentiometer. Its in mm now.

The original STL files came from here: http://www.andymark.com/product-p/am-2618.htm

I used a different 10-turn potentiometer, a AMM20B5A1CLASL380 that is contactless hall-effect sensor with rotation life 50 million shaft revolutions. https://www.digikey.com/product-detail/en/bourns-inc/AMM20B5A1CLASL380/AMM20B5A1CLASL380-ND/3868206 But you can always use simple wirewound potentiometer like this one here: https://www.digikey.com/product-detail/en/bourns-inc/3590S-2-103L/3590S-2-103L-ND/1088586

For the spring I used one from cheap ID badge holders like this one: http://www.ebay.com/itm/10-x-ID-Card-Holder-Reel-Retractable-Badge-with-Belt-Clip-Ring-Pull-/371255688124

See a video of it in action: https://youtu.be/3MMJfBPJwKc

A new video with better print and some tips here: https://youtu.be/NsgG6o2HFr4

Thanks Thanos