On Sat, 27 Feb 2016 03:58:02 -0800 (PST), bob_villain
wrote:

On Friday, February 26, 2016 at 11:38:03 PM UTC-6, wrote:

What about pop rivets?
Even short pop rivets protrude into the duct, but a real short rivet
is better than a screw.. The rounded end of even a longer rivet isn't
as bad as a sharp sheet metal screw tip - but is not the best idea.

...and the small tapered end of each section should face toward the outside, or lint will catch at the connection.
That , I assumed, was a "given" - ductwork is ALWAYS installed "into"
the "downstream" part.by default. The duct out of the trier fits
"over" the stub, and you just keep going. You'd almost have to TRY to
do it wrong - but then again - - - - -