As an alternative, why not just the standard mod?
Peter in Hong Kong did my 450D
peter@tan14.com
http://www.tan14.com/gears.htm
PS its not mono, the service he provides is just removing the IR filter-but if what you are after is narrow band you can still do use your DSLR (though its not as efficient as you throw away a more pixels than you use. Just expose longer
)
still works ok for narrow band imaging with a H Alpha or O111 filter. Never tried a SII