diy decorations help

[nismo300zx]

ive made the styrafoam and concreat background. but now im wanting to make my own decorations. have a plan to make a dragon the length of my fish tank. but am unsure about the medium to use. maybe the quickcreat, but its not as smooth or detailed of a texture. and clay will eventually soak up water and more than likely kill my fish. anyone have any ideas or tutorials of how to do something like this? preferably a cheap way of doing it.

thanks,
S