Another option to remove the noisemaker and clean up the engine bay is to get the OE production CAC tube from Ford for the 2015-2018 Ford Flex. This will completely eliminate the noisemaker without any need to install plugs that may pop off over time.
https://www.tascaparts.com/oem-parts/ford-outlet-tube-aa5z6f073c (https://www.tascaparts.com/oem-parts/ford-outlet-tube-aa5z6f073c)
For $150. I would just get the Ecopowered Part. But thats just me.
https://www.ecopowerparts.com/shop/index.php?main_page=product_info&cPath=1&products_id=11 (https://www.ecopowerparts.com/shop/index.php?main_page=product_info&cPath=1&products_id=11)
That's why I dropped my price to that $150, our part will never fail, the factory one will eventually over time due to the rubber hose.