While I was building my SS7 i had the same question. How to make the tubing make a 90 degree turn below the floor board and pass through the fabric. John McBean explained how to put something like a large gauge copper wire in the tubing to keep it open, heat it carefully but dont kink it, and bend the tubing and wire to 90 degrees then remove the wire. With little or no experience with this I did it, I had a 90 degree bend, the tubing remained open and it has worked very well for 106hrs now.