My boat has a vent in the forward part of the cabin that we leave open all the time. I think is the Beckson C-6 Vent O Mate (4" hole required) which only costs about $26. Mine has the optional (another $26.) stainless cover which is probably needed because the plastic one is too easy to break.
It works well at keeping the boat ventillated when closed but you'd want a couple of these to make it confortable for sleeping when everything else is closed up.
I haven't checked, but both of these items may be available here on Sailnet as well.
Another Vote for the Beckson vents. I installed two on our present 34 footer, and previously put one on my 26 foot sailboat.
They work great in the slightest breeze, 24/7, and have no electrics to fail like the spendy motorized ones.
If you take green water over the top or are just washing down with pressure water you just close the vent from the inside
and the spray is kept out. It is deceptively-simple engineering, and well designed for the purpose, imho.
I also have the sturdy ss tops, after having the plastic one break on our prior boat after years of UV exposure. Of course dropping a spinnaker pole on it did not help things much.