I guess you have a coldframe from what it looks like. Its very nice! Using a double plastic or double windows on the top will work better to hold the heat. A little tip that I am doing this year is putting several gallon or quart jugs of water in the corners which will help hold heat from the day and help cool it when its too hot. One of the biggest probems is if it gets too hot especially when the outside temp is above 50 with the sun out. .
Another thing is paint the back wall white to reflect the light back onto the plants. I have about 8 or more cold frames of all sizes. I use one for a compost that I will cover with a foot of dirt more near the spring and plant my first lettuce crop in! All my cold frames are basically raised beds that I use for both cold frames in early spring and raised beds in the later spring!
I enjoy fishing ,gardening and a solar greenhouse! carpet installation repair and sales for over 45 years! I am the inventor of the Bobber With A Brain - Fishing Bobber!