you need to get a bolt or such with a long shank, with correct threads (not sure of size) and use DuBro large prop method, see vid at 11:30.
[youtube]http://www.youtube.com/watch?v=OXuNnYQO2s4[/youtube]
or another method, but use the threaded motor top.
[youtube]http://www.youtube.com/watch?v=89Judpr8ILo[/youtube]