How to create custom widget element?

Hi! I try to create Double Slider (as on pic)

114940-doubleslider.png

by inhereting it from usual Slider, but get no luck. Can anyone help me with this?

DoubleSlider.cpp and DoubleSlider.h in attachment zip folder

link text

Hello, does it work on 4.26 or you happen to have an updated version? Thanks