Here are some drills that the paper from brazil turned me on to, They seem simple and easy to build, and the second link is a simple roller that was designed for that same project.
lots of interesting stuff on the iapar website even if you cant speak protugese
http://www.iapar.br/arquivos/File/folhetos/tracaoanimal/tracaoanimal.html
http://www.iapar.br/arquivos/File/folhetos/queixada/queixada.html
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