I picked up a plastic welding kit at HF. It is just a soldering iron with a larger head on it but it was cheap. Then I trimmed up the duct that I had already made and welded the two sides together. As I said last time it's not pretty but no one outside of this page will ever see it.
Also, I am trying to install the wiper fluid line before I install the plenum. I am running a line from each side to a tee but where does the 3rd line from the tee pass through the firewall or does it really matter?