I have one of Steins and it opens in flight. It actually opens on take off and closes in cruise. I plan on taking it apart and seeing if I can fix this.
Funny you should bring this up. I just replaced the originals with metal vents by Air kit llc replacements. The originals are junk. I bit the bullet and spent the $300 and am very pleased. They seal tight and can be easily adjusted for firmness. A small price in the long run for a more comfortable cabin.
I also had a hard time spending the extra money on the Steinair vents but sure am glad I did. Never an air or water leak and they remained in the position I set them at under all flight conditions.
Aerosport Products offers aluminum Aerovents. These vents come with the option of either the ring nut or the 2" scat tube adapter. These vents are very high quality with no air leaking when closed. We also offer a aluminum adapter plate for the stock Vans square plastic vents at no charge. The Aerovents vents are $115.00 each. If you buy a Aerosport instrument panel or overhead console the cost is 105.00 each.
It amazes me that there are so few moderately priced products in this category. You swing from the cheap plastic vents from Van's to $100+ aluminum vents.
Why someone can't produce a $25 plastic vent that seals effectively is beyond me.