Can anyone advise me on what icing to use for the following decoration:
I am planning on making a cast of a favourite toy (about 2 inches in depth, 12 inches long, & 4 inches high) for the top of a child's birthday cake, in scilicone. I then want to fill it with some kind of (royal?) icing to create an edible model for the cake top. Ideally I would like a pourable icing to create a truely 3D cast, but am afraid that just royal icing would not set hard enough, or indeed at all in the middle. Could anyone advise me on what could possibly work in this situation!?

Actually you could use the royal icing. Just pour enough in the mold to cover and pour the rest out. Let that harden then keep repeating the process until it is the thickness that you want. That way eveytime you add a new layer the previous one is already dry. That should work and you won't have to worry about the middle not setting up completely. Does that make any sense?

I don't know the answer to your question, but have you thought about making it out of the fondant of your choice? It might be easier.
I'm sure whatever you do will look great.

Many thanks Brendansmum, Never thought of that, I'll give it a go & let you know!
Bulldog ... I'll might try fondant if the royal icing doesn't work out.
Thank you both for your help.

What about sugar? You could boil it until hard crack and pour it in.

Or you could make it like they do those sugar Easter eggs. Just mix a bit of water into sugar and use it like wet sand to make a sculputure, and it will dry hard as a rock. There are directions online for it.

What about candy melts?

What about candy melts?