You are asking a visual question or are there matters that set how the long items behave on the conveyor? I would set the item size much shorter than the visual appearance of the shape. This means the attributes relation is:
item.attrs.spatialsx.value << item.attrs.offsetsx.value
Then I would set the visual location of the shape in relation of the item dimensions in the way that the item is at the back, the front or the middle of the shape, just as you need the item behave on the conveyor.
You only need to increase the gap between two items on a conveyor.
If it really important that item shape looks nearly realistic you can change the shape location relative to the item dimensions dynamically.
