Iguanas love to get up high if there are upper shelves and paths for them to climb up. They are mentally healthier in that kind of cage, and the exercise they get from climbing makes them physically healthier. It makes gravid iguanas dramatically more successful in laying their eggs.
Passive ventilation is enough. Forced ventilation is too much. Let the warm air passively rise up and out of the top holes and pull in fresh air at the bottom. Too much ventilation will lose the humidity and heat your iguana needs so critically.
I think this habitat only needs three 1/4 inch holes at the top and bottom. Tape over the rest of them. Plenty of air will be sucked in and blown in and out of them by the heating system. Besides, iguanas aren't exactly oxygen furnaces, they have slow metabolisms. And if there is no water condensing onto the interior surfaces of the hab, there will be no mold problems. Bowls and tubs of water are OK if you want them.