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 separated, sandwiching the dash between the top and bottom sections, through a 33mm x 168mm hole.

It will accept our 55mm lightweight ducting #DCTHS55.

Two, counterbored 5mm fixing holes at 183mm centres.

Overall height 85mm. 

DEMIST VENT 206mm x 46mm #VENT14