layout:Column-cols and padding in HBox

[RudolfVonKrugstein]

I want my elements in a HBox to be arranged using a percentage I specify with "layout:Column-cols", but with empty space between them. So I try:

 <HBox widthPt="100" heightPt="5" layout:Column-cols="[0.2,0.5,0.2]" padding="20">

Unfortantly the padding paramter seems to have not effect. What's the best way to get some space between the elements in the HBox?