HUD boxes for SceneObjects

We want to be able to put items (t2dSceneObjects) from the playing field into boxes, which should be HUD elements. So you click and drag one object, move it over the HUD and release it. It will be some sort of inventory, but with no fixed spaces. You can put items into the box and put them back onto the playing field.
The HUD boxes should be resizable and able to be set to invisible.

I played around with the GUI controls a bit, but I haven't figured out how to achieve the above mentioned. Does anyone have an idea?