Enerjet by Aerotech, Arreauxbee-Hi, Part 11, Channel Tunnels

While the instructions say to cut the tunnel extension pieces at 1 3/4" long. 

I cut mine 1/16" longer so the final length ended up at 1 13/16".

I used the aluminum angle to align the two pieces.

Here you can see the piece extended beyond the end of the tube.

Sand the extension down even with the top of the tube.

This ensures the tunnel is full length to the top of the tube. Sanding down the tunnel with a flat block will make for a better "mate" of the upper tunnel coming up.