http://www.harken.com/productcategory.aspx?taxid=362
http://www.harken.com/uploadedfiles/Product_Support/PDF/4059.pdf (Installation Manual)
http://www.youtube.com/watch?v=atITPE_Ld08 (Shows you how to install it.)
We used this on our Sabre 42 years ago. We simply eased it slightly when we were sailing if we wanted to pull the boom down with the vang. We continued to use the same sail cover -- i.e., we didn't modify it to hide any lines.
I'd call Defender and order it -- or look and see if Hamilton will "do a deal" for you. If you want to duplicate it, I don't think you save much money.