Your new "greenhouse" looks good, but I don't think that is what you are looking for. What you need is an "overwintering" house, completely different. Your greenhouse has been covered with clear poly to let in the sunlight. That is what you DON'T WANT! Greenhouses have been designed to let the light in so plants will grow. You want to simply keep your trees protected from the wind, a bit from the cold and more importantly THE SUN. You don't want the sun in winter. Yes, even the evergreens, you want DARK.
During the past 30 or so years nurserymen overwintering container grown plants have demanded "overwintering poly" which is white and opaque, which does not let in the sunlight. The sun will raise the temperature of your greenhouse and create the plants to break dormancy too early. NO, unzipping or making slits in the poly will not do. In fact, it may cause the poly to rip, especially in windy locations.
I've been overwintering bonsai with poly for nearly 50 years so have had some experience with this situation. I made the same error decades ago.
Now, I suggest you get some cheap paint, (discontinued colors are fine) and paint the poly on the outside. A light color is best, but some of my students tend to paint their poly to match their homes so it does not stand out too much. It's best to wait for a rain or two because the poly often has an oily film which repels paint. Just paint the poly, it will defect light and your trees will be much better off, especially in February and March when maples will want to leaf out earlier.
Next season, look for "overwintering poly" which is milky white and can be found at a nursery supply company.
Just a bit of unsolicited advice from someone who has been there and wishes someone advised him what not to do. Good luck, let us know in spring how you made out.