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Don't take the tubing since there is no room for it in the back-pack.

Well, now that I have a sufficiency of string, can I make a helmet and then have room for the tubing?

You made a helmet, but still don't have room for the tubing because all 6 spaces are still containing items. You would need to throw out something to take the tubes. Look at the inventory and you will see that all 6 inventory slots are full.