The light thru the plexi should be enough. You'll want to vent to get rid of excess heat, when on a 50º day you can get to 90º if you aren't careful. Many of us use an auto vent opener to overcome this issue, which for me in Denver area at ~5700 feet is a real issue, even in Jan-Feb. Two layers of plexi with a ~10-20mm space should give you much, much better resistance to heat loss. Closer to 10-15mm is best, but depends on your tools. A couple dabs of silicone to maintain distance between panes is what you want.

Dan

I wouldn't expect light to be an issue for you, but venting is. You'll have plants in the frame accustomed to low temps, you don't want them to get exposed to intermittent heat by not venting.

There's a rhythm to growing in frames: you plant stuff for fall/winter in fall, then you can plant stuff for spring in winter (they'll come up when thay can), and stuff for summer in spring. I don't quite have a handle on that yet, my winter sowings of spinach have not done well.

I have carrots in the frame now that were about pinky-finger size before our weather turned cold, those were sown in mid-september.

Keep in mind that plants will probably not grow actively in winter, but if you start them earlier in the fall, they will keep in winter just fine.