I would suggest NOT doing this; it is not natural and you will cause more damage. (Dereck shakes his head) Instead I would suggest looking at "Stretching Scientifically" by Thomas Kurz.
While I have not done everything this book has suggested, what little I did help considerably for my flexibility when doing Taekwondo.
In all seriousness, DON'T even consider a stretching machine and especially not one you may try to design yourself. Leave that stuff for the movies where it belongs.