Glass beats plastic for the simple reason that it is far denser for the amount of light let in. But yes, the plastic would be easier. You could use floating row cover to add insulation. Drape it over your cage, then drape the plastic over. This will add 4-6 degrees of frost protection, depending on how much of the base is covered - the more of a triangular shape you achieve, the better.

Nice work! That looks awesome! It looks as if your cold frame is facing to the east, with the back of it to the west. correct me if I'm wrong. At high noon the cold frame should have 100% direct sunlight inside, over all the plants. Yours looks just like mine, same plastic and all. The back should be on the north, IE, the cold from should slope downwards toward the south. However, there should be plenty of light that gets in the cold frame through the plastic sheeting. I know it doesn't look like it, but the dull light is perfect for brand new seedlings (like a week old, yours look older than a week) At the bottom of your last picture, the lettuce looks like its getting good light through your plastic, is the wooden frame itself shading the other guys in there? and to answer you about hardening off, seedlings only take me at most 3 days to harden off. I leave them outside in the cold for an hour and then bring them in, and the the next night increase the time by a couple hours. By the third night if it isn't too cold (like below 45) I'll leave them out all night. In a cold frame they should be well ready for a night outside in a windless cold frame. The real bad thing about hardening off isn't the green leafy part of the plant getting cold, its the roots, which are susceptible to shock. soil temperatures, even near the top inch stay pretty even keeled, and change gradually. leaving a plastic potter out overnight meant the root system has to go from 60 or 70 degree soil to 40 or 45 degree soil in the space of a few hours.. its a tough trick to turn if they aren't used to it. Lettuce will germinate right down to freezing temperatures, its incredibly cold hardy. while it takes months at 33 degrees to germinate, the plant itself doesn't need to be hardened off, at least not cautiously so. give it a couple hours in the nighttime cold and then they'll be ready to stay outside. I've never lost a lettuce seedling before, and I've forgotten more times than I can count. I did lose a couple tomato starts to shock one year though... Which part of zone 5 do you live in Chele? If I may be so bold as to ask. I'm on the southern edge of 5, almost to zone 6. Sorry for being a chatterbox. Here is a link that might be useful: My blog, if anyones interested

Personally I prefer to use black plastic for warming the soil. I've found it heats somewhat slower than clear, but it also doesn't allow weed seeds to start growing either. I usually lay it down about 2-3 weeks earlier that I plan to plant and the soil is warmed sufficiently enough. Also I normally cut holes thru the plastic to plant so the soil continually warms for a few more weeks. I'll remove it when I start laying down organic material for mulching about the fist week of June. I only use plastic mulch to warm the soil for: Melon, Pumpkin, Eggplant, and Okra so I can plant out between May 11-15th vs waiting the extra 2 weeks after the last average frost date has passed. One year all I had available was clear trash bags to use. I laid it over the hills I created for the melon. Within about 1.5 weeks I had well over 1,000 Amaranthus 'Intense Purple' seedlings literally covering the hills LOL! I took this picture while I was in the process of laying down mulch; as you can see I ultimately decided to allow some Amaranthus to stay, but many of the smaller ones ended up in the compost pile. No more clear plastic after that fiasco :O PS. How soon you want to lay it out will depend on your soil type. Soils having a lot of clay will warm up much slower than a sandy loam soil :)

Yes, go for it. there are other possibilities, but your idea is cheap and sensible and a good size. Now that frosts are not so common, you will only need them on an occasional night, when the air is still and the sky clear. Don't mulch your soil at that stage - let it absorb the heat of the sun during the day and radiate it back into the bucket at night.