DIY/handicraft question

Wow Ice, people have had some brilliant ideas here 

Well some people  anyway 

When I was a primary school teacher we used a strong corrugated plastic sheet that was easy for the children to cut and score, to make models. We added lights, motors etc that were controlled by the computer. It comes in various colours and that very large handicraft store stocks it. I'm sorry but I cannot remember it's name or how much it costs, but it seems to me that it will do the job for you.

Another alternative:

If you can get hold of some polystyrene bits of packaging to cut the discs from, a couple back to back should do it (depending on the thickness of expanded poly you can find).  Generally strong enough to support most non lead-based cakes.

Then for the edge a small amount of the polystyrene underlay they sell for laminate flooring with the metal layer peeled off.   Would only require a tiny offcut (about a yard long and whatever width you end up producing - much less than a single 'fold' of the stuff.)

bostik or any of the old fashioned modellers glue will do to join the strip to the outside rim of the circular block.

Between the layers of polystyrene in the disk part it is easy to scrunch long dips for channels to contain wiring etc.   Before fitting the outisde layer you can mark it up with the pseudo-milling marks and the 'DECUS ET TUTAMEN....' or whichever you want, if the 'giant pound coin' is the effect you are after!