I have never tried this myself, but you can try to make a dummy using Crayola Model Magic or a similar product. It's a modeling clay that dries and it super-lightweight. You can press it into your pan, maybe 1/2" thick or so and let it dry, then pop it out and you'll have a shell resembling your character pan. Try it on a pan you don't care about, because I HAVE NEVER DONE THIS, so I'm not guaranteeing that it'll pop out when it's dry! 
Just an idea.
Two suggestions:
use the pan itself (or a duplicate). You can pipe the stars right on the surface, stick it on a board with double sided tape, and pipe a border--good if you need a short-term dummy.
Second, mold rice krispie treats into the pan. You can even make a fondant mold of the surface and overlay the rice krispie treats so that you can see the imprint better.
Good luck!
Rae
Hi,
This is another idea.
I just read somewhere about making moulds with paper machê. You soak torn pieces of newspaper paper or any absorbent thin paper in a solution of wall paper glue which you buy where they sell wall papers. You make the solution with 3 times more water than given in the bucket. When the paper pieces are well soaked oil your mould and cover it with the paper pressing to make sure that the corners are properly covered. Once it is dry it will come off the pan.
I haven't done this and I do not have character pans to try but in a few weeks time I need some round and square cake dummies and then I can tell you how it comes out.
A really long time ago, Wilton had instructions for something called a "sugar mold."
Made from like, I think, sugar and just a bit of water. Mixed until it resembled wet sand, then pressed into the pans. Pops out and is rock hard. Lasts for years, if a bunch of moisture doesn't get it.
Here you go...
http://home.inreach.com/marthac/craft2.html
HTH,
--Knox--
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