Here is what I believe is a great and affordable soft jaw / clamp / holder for bicycle dropper posts (or rigid seat posts if you are into that kind of thing). 

Item Specs:

• Sizes included: 27.2mm, 30.9mm, 31.6mm, 34.9mm
• Two 12x3mm Neodymium magnets per half - quite strong
• Total width around 160mm / 6-1/4"
• Magnets are around 80mm / 3" apart
  
• 3D printed using PETG filament, a stronger alternative to the commonly used PLA material. Print settings built around strength (high fill% settings, thick perimeter walls)
• Material is soft enough to not damage your beautiful seat post finish, strong enough to hold it tight.
• Halves snap together with magnets for easy storage (see photo)