This section can be of any length, enough to incorporate your flow straighter, the hole for the gas intake, and viewing screen.
For this section, what we did was cut out 3 parts of the 4 and for the 4th part, use a shorter piece cut that in half and have the Lexan sheet in the middle of the two. Don't forget to peel off both sides of the Lexan protector!
The last two images show how to make one of the two stands used to stand the tunnel up since it is at an angle without this part because of the contraction section. Just cut slots into each piece and shove them together to make the stands.