If you had a stem like that it wouldn't take much to fabricate one. Buy a sheet of carbon fiber (or aluminum if your cheap. Cut to size. Glue the 500 mount on, and you've got it.
Heck if you're really cheap. Cut and fold a beer can.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.