I put metal fins slanting down on the outside, if you attach an old flexible 2 inch pool filter hose with self tapping screws on the inside then put a bend in it, no light can enter if a spearing shack. Then cut an old foam pool noodle like a large cork and you can plug vent when enough fresh air is in. Put cork on a string and attach to pool hose, so never misplace foam plug. (2) I also put some in without the hose, pull foam plug and put your eye close to fins and you can see tipups outside.
