I think you pretty much have to use acrylic - oil would crack and watercolour just wouldn't be suitable. I think if you're worried about it crackign it's worth mixing it with the stays-damp-longer stuff, which has a fancy name I can't remember at the moment. It makes acrylic much softer and more malleable and gooey when it dries, so it shouldn't crack. You should be able to get the stuff from a good art shop.