This chain-keeper tool is a easy to use tool without getting dirty hands. Just move the derailleur aside and pull the chain from the cassette. Now you can remove the wheel and the chain stays in place.

Last image: shows just a few redesigns towards a working tool :)

Use some lubrication on moving parts. 
 

Used springs:
Below the M4 knob : D=8mm, L=30mm, wire=0.7mm, pitch=4mm
Ball lock; D=8mm, L=11mm, wire=0.9mm, pitch=4mm
On the M6 bolt: D=9mm, L=18mm, wire=0.7, Pitch=6mm
Different springs may work to, I used what I had lying arround.

Used the commercial “chainlift” as challenge to get familiar with OnShape.

Check also my other designs: https://www.prusaprinters.org/social/201278-hans/prints