Don’t know what type of vent you have. I had a Plastimo dorade vent on my coachroof which I removed and replaced with a solar vent. The hole needed for the solar vent was about 10mm diameter bigger, so I had to enlarge it (taped a carrier bag to the roof on the inside to stop dust going into the boat, and then used a drill with sanding wheels to enlarge the hole.
The solar vent I used was a Chinese one from ebay, and about 5 or 6 years on, it’s still going strong. I have it set on extract, and it works well to keep some air circulating through the boat when it is closed up. The solar fan cost about £40 if I remember right.
Yesterday I scanned all the various sources and found that the depths available did not seem to deep enough to fill the space between the deck and headliner of the inside. Did you have the same problem and if so how did you fix?
What I did was got a round vent from Screwfix/Toolstation (meant for an extraction/cooker hood type thing) and fitted that on the inside to cover up the hole. You wouldn’t know that it wasn’t original fit.