Do the shims. I put the tubs directly on the flexwatt and after a year the feet on the tubs wore down the flexwatt enough for it to shock me when I touched it.
I have a whole write up on how to wire flexwatt, but you can see pictures of my set up and the shims. http://www.slitherinsisters.com/how-...e-flexwatt.php
As for the ventilation as long as your tubs has some sort of feet there will be some airflow.
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