How can I control the placement and size of elements in a group?

I would like to have the behaviour of the UI containers but with my own containers: when I resize the container the buttons and top bar remain the same size so they are positioned relatively to the container but the size is fixed when I change the container in size.

I have been looking for solutions like but have not found any:

- defining a shape as a container.

- changing the default shapes and add my own buttons.

As a functional analist I often make proposals for the gui and a solution to this pain would help a lot!!

Thanks in advance 

Sara