Ok, in my last post, I bemoaned the prospect of cutting the semicircle parts of the skins (8 in total) without making a total mess of it. I have a cunning plan that may be the solution. Years ago, my wife was involved in Creative memories (scrapbooking, blah blah), and one of the implements she used was a Circle cutter, which allowed her to cut out circles of any size using an orbital tool (pictured below)

All I have to do is find the central point of the circle, and voila, the blade should be able to score the Styrene pretty easily, and this should hopefully allow me to either cut through fully, or enough that it should be able to be snapped off. Tomorrow will tell.
These are available on ebay fairly cheaply, simply search for Creative Memories Circle Cutter