If you are determined to make one yourself, there's an old tried and tested method not much used these days. You would use the casting as a plug, waxed first then overlay thin strips of gummed paper strip. After a thick enough moulding is made, finish it with shellac / varnish to stiffen it and remove from the plug.
I've done this years ago, it's fiddly and laborious, and takes a lot of effort with the last internal and external strips to simulate planks, but it is realistic and surprisingly strong.
Chas