2 different issues - hardening off lettuce seedlings and how to use a cold frame.

Planting in a cold frame is very different from using one to harden off seedlings. 1) Seedlings in containers don't get the benefit of the warmed daytime soil to see them through the night like direct seeded plants do. 2) the direct seeded plants wouldn't even have germinated yet and when they do they will already be adapted to the conditions.

So since you still have snow on the ground and so cold soil and you already have seedlings raised inside then just keep them as is until a week before your transplant date and then harden them off normally.

Trying to do them now in a cold frame, no matter how well it is constructed only over-complicates the whole process.

Had you already set up the cold frame a couple of weeks ago and well warmed the soil already then yes you could direct seed in it now with no problems other than having to monitor the air temps inside and open for some venting in the sun.

Alternative - learn about Wintersowing techniques on their forum. Easy, works great for leafy greens, and no adjustment period needed.

Dave.

I planted lettuce, spinach and Kale in a cold frame in October. Some did well and some dried out because I could not go out and water often enough. ( too much snow)
Prior years I did much better. I was picking lots of spinach in March ( zone 6b)

Look for varieties that are more tolerant of winter weather. Look for the book -
Niki Jabbour - The Year Round Veggie Gardener
She lives in Nova Scotia --- good luck

Here is a link that might be useful: Garden Design Ideas

That's what I told DH - no need for cold frame now, I was afraid the greens would get too cold at night. And I'm not starting peppers and tomatoes in one - still have snow in the areas where I want to plant those even though it's melted off the garden area near the house where I plant things that need more water.

My plan is to just harden off the greens like I do nightshades, bring them out for a few hours, building up to full sun, when temps are in 40's and 50's, but put them back in basement at night until nights are above freezing, then start leaving them in the garage and eventually mid-April or so when ground warms up, transplant them. They are just starting to get true leaves. Cold nights put row cover on them.

Now if he wants to build some cold frames for the fall, I'd like to extend the season. Then I can sow directly in them for next spring.

In fact, I suspect that what he really wants to do that he's calling a "cold frame" is to put up some hoops over the lid to the septic tank and put some flats/pots there since he talked about doing that when it was the only bare spot in the lawn - he said we might as well take advantage of the heat. I think he just doesn't like me having racks of flats/pots in the house for 3 months ;-)