Here's a nice two-piece vent that can be fitted either as an assembly from the top of your dash through a 38mm x 172mm hole or seperated, sandwiching the dash between the top and bottom sections, through a 33mm x 168mm hole.
It will accept our 50mm lightweight ducting #DCTHS50.
Two, counterbored 5mm fixing holes at 183mm centres. Overall height 85mm.
DEMIST VENT 206mm x 46mm #VENT14