I think this would be excess
I don’t think there another way, tbh. Create a user widget with the desired size and implement the onDrop there. Other than that, you would have to manually calculate relative position inside the widget. Doable but seems over the top.
Depending on the scope / complexity, I would add a native Named Slot widget and fill it with a onDrop handling child. This will at least give you some modularity and design granularity.