10 square inches of incoming free air ventilation and 10 square inches of outgoing free air ventilation is the minimum.
The vents should be placed 12 inches from the ceiling to the bottom of the grille high on the wall and 12 inches from the floor to the top of the grille low on the wall.
I want to say to please go ahead and get a CO detector with alarm regardless of how you ventilate. Ventilation can be somewhat unpredictable inside a busy little area such as your fishing hut.
I hope this helps. Please let me know if I can be of further assistance. Thanks for using JA!