26th June 2009: Top Hat progress.. I applied some filler to smooth out the steps at the top, now has the fluted shape I was after. Using a lollipop stick and some of this 'Lightweight Coving Adhesive' (left over from a DIY job, this stuff is meant for securing the lightweight polystyrene type coving) - I found it is an excellent lightweight filler for cardboard. It is easily sandable, sticks really well, is premixed and dries really fast (so doesn't soak the cardboard).
The filler is now being left to dry thoroughly, then I will sand it to a smooth finish.