So here I have given a basic wyvern conversion so it can easily be swapped between a wyvern and hydra.
I started by assembling it as a wyvern. I then used old bluetac and put it in each of the barrels. Following that I drilled holes into each of the barrels and put in brass rod as well as shaved the sides so it could go in. The last picture shows how it looks with the barrels in.
In addition here is a few pictures of the painting in progress