I used my 3d printer to make some extensions for my son's stroller, which is too short for me.
This is the 3d model of the clamps
This is the model for each knob. I can make them for different sized holes and nuts by scaling the yellow cylinder and the green hexagon before subtracting them from the knob.
These are some prototypes I printed. I first tried using SketchUp to do this and came up with the ugly prototype on the right, since I don't know how to do much in SketchUp that doesn't look blocky. So I decided to learn Blender to do something more rounded and organic looking, and that's how I came up with the shape on the left. It also has a hole in the middle with some material removed to save on plastic.
The knob on the top is also made with SketchUp. The knob with the three prongs below it I made with Blender. I decided to go with 4 pronged knobs in the end since the symmetry is much easier to model in that case, at least with my limited skills.
These are the final pieces printed in white PLA plastic, before painting.
And the final result. I used cement glue to stick the metal nuts and bolts to the plastic and an angle grinder to grind off some of the extra length of the bolt. I made the actual extension from a piece of 2x4 lumber and a 1 1/4" pine dowel which I spray painted black.