My first choice would be to use twin walled polycarbonate panels. Since your greenhouse has a wood frame, they would be easy to install. You can get them in 2 x 4 panels and put in your own vents. Twin walled polycarbonate would also help to diffuse the sunlight hitting your plants.
If you can't get them locally, Greenhouse Megastore has them starting at $16 a panel. They also sell the fasteners and sealing tape.