updated 04/28/20 Click on the image to open a 3D .pdf version of the design.
I have modified the standard Center Vent Surrounds to have a set of Fins that act as depth stops. The make it easier to position the vents so that the correct amount of the vent extends through the skins.
You can download models of the standard vent surrounds in STEP AP203
(.stp) format in English (inch) units as well as in 3D.pdf format: