TrampSteamer2.pdf has a whole pile of great information on interwar tramp steamers, including some diagrams that show the cranes/derricks/spars/etc. Some further links and a bibliography as well.
Google Image Search can also be useful.
One thing I've seen done, and intend to do myself one of these years, is use old paintbrush handles for the spars and booms - that gives a nice tapered shape very easily, and paintbrush handles are usually hardwood of some sort that takes drilling and such easily.