Hi, I want to make a RKT figure cake topper. The cake is due 30th July, how far in advance can I make it?

I would mould the RKT figure, cover it with royal icing to get it smooth and then use fondant to cover it and add detail.

I want to make a figurine of Stewie from family guy, should I make my own RKT mixture or can I use store bought? It won't be a huge figure and wouldn't want to make too much if it'll go to waste.

I am from England so would be using something like this if I use store bought:

http://www.kelloggs.co.uk/products/ricekrispies/Bar/rice_krispies_squares_chewy_marshmallow.aspx

Thanks for your help!