I did a cast of a snake with alginate. Just lay the snake out how you want it positioned, and use pins to stop it from moving when you pore, and pore some alginate over it.
You may want to use clay around it to act as a damn so it doesn't go everywhere. The alginate will go off really quick (2-3 minutes) so you have to be fast.
After it goes off, just flip the mold over and slowly pull the snake out. Now you just mix a 50/50 mix of bondo and resin (with both hardeners added) and pore it into the mold. After it hardens sand all the scale detail off and your ready to go.
This is one I did. As you can see, it picks up a lot of the detail.


Here is his head.
