Create a parametric cell for labels

Hi,

I am trying to create a parametric cell for a label.

My label needs to contain 3 text fields that are linked to items or element properties. These text fields are separated with vertical lines.

I would like to constrain the size of the box to the lenght of the text.

I managed to build this cell using constraints between the edge of the rectangle and the edge of the text. 

But when I place it using place label tool, the constraints don't work, see example below :

If I change the cell from graphic to parametric, then the cell is not available anymore in the drop down list for labels.

Do you think it is possible to achieve what I am trying to do ? Or am I stuck with fixed size label ? 

Thanks for your help.