Radon barrier around telescopic air vents

[Nwhent]

Hi, I am a bit stuck with what to do when installing a radon barrier around a telescopic air vent.

 

We are first time self builders (so sorry in advance if this is a straight forward thing) and we are trying to do what we can ourselves. We are taping down the radon barrier now but below our bi fold threshold we have a number of telescopic vents and I am not sure what is the best way to make a tight seal and any advice and tips would be of great help.

Thank you!