Freeform Hotstops - Drag and Drop Issue

Hi,

I've created a desk drag and drop activity which involves a graphic with three arched zones (see below).

The learner needs to drag and drop desk items onto the three zone (rare, occasional and usual).  When the object is dropped they see a tick or a cross to give them instance feedback.  Only when all are placed correctly can they move forward.  

The problem is I'm getting false positive results though when the item is moved onto the inside of the arch of another hotspot.  If its dropped incorrectly and then into that middle area of the correct answer the learner gets a tick and a cross.  I'm guessing this is because of the shape of the object.  I've tried hotspots, transparent freeform object and even an image with transparent background but they are all effectively the same shaped object (a rectangle).

Does anyone has any suggestions?  The image doesn't really allow for the use of straight rectangle hotspots as some objects will be normally be arranged on the side of the desk (notebook, telephone etc). 

I'm guessing I'll have to make it a submit interaction and just give them feedback but thats going to be hard to do well.