For a hot house, you don't really need insulated panels.

Just cover it with a blanket when it gets extreme cold those few days.

dcarch

I do need some regular coverage, I think. I start my seeds in-doors around now, but than around mid-March/April. I will put them out int the hot house. It does get cold at night and and will rain from time to time. When it doesn't rain it would be great to have it hotter inside the structure than the outside temp. The blanket would make it hotter but not let my light in. Without it would be chilly.

Plastic? 6 mil?

Try the "so-called clear" corrugated fiberglass patio cover panels. You can buy foam rubber gaskets to fill up the corrugated spaces. The are a lot cheaper than plexi, Lexan or polycarb. They will last about 5 years before they get yellow.

There are a number of vendors that sell plastic film, 4 mil stuff that will last 4 seasons. It has UV protection and let's a high percentage of sunlight through. My greenhouse is 12" x 10". I ordered one piece to go from one side up and over to the other and then individual pieces big enough to cover a frame on both ends. I don't know how to attach a picture to this post, otherwise I would. Google greenhouse supplies and you will find local suppliers of this greenhouse film that you can order from. I just built a frame one end with a door the other without out of 2'x 4' and 2' x 3', attached the film with lathe board and small nails. If I was a better carpenter it would be nearly air tight. I use a small heater that has a thermostat built in and set it to 65 degrees. So most of the times at night it will keep the temperature above 50 degrees. But I live in the high desert east of Los Angeles and it rarely gets below 25 degrees here.