Sounds like an interesting project. I wish I had your construction skills! I would love to see pictures of your project when you get it going. I'll just answer some of your questions based on my own experience.

1. Definitely slope the sides, because wet snow, and even high winds, can pile up and easily bend or break metal frames, much less wood.
2. I don't like the idea of permanent vent holes. During periods of cold weather, and definitely at night, those vent holes will mostly cancel out the benefits of having triple wall glazing. And during warm sunny weather, those holes will not be enough. At the very least, I would drape some row cover over the frames at night, or just manually vent during warm or sunny weather.
3. In my experience, an air temperature gain of 15 degrees definitely does not translate to a 15 degree rise in soil temps. more than an inch or so below the surface because the soil is constantly losing heat to soil outside the frame as well as to lower layers. For example, in my greenhouse that I use like a cold frame (with row covers inside), the average daily temp. is about 15 degrees warmer than ambient in the summer (as I ventilate during the day). But the soil at 6" is only 2-3 degrees warmer than outside soil.

4. I would go with reflective fabric on the north wall.

Agreed about the vent holes. The automatic vent openers would do the trick nicely, opening wide when it's warm and staying closed otherwise. Plus, they don't require any electricity and you don't need to be around to worry about opening and closing them.

Thanks for the thoughtful responses. I guess I was trying to minimize the construction time, complexity, and maintenance. But it makes sense that vent holes would quickly dissipate warmth when the sun goes down and might still not prevent cooking on sunny days. The sun is quite directional in early spring, another reason to slope the cover.

Sounds like I should experiment with a few designs, including side vents and openers, but should definitely slope the cover and keep the frame (when the vents are closed) airtight.

Yup, it sounds like you are on the right track. Those are the same vent openers we have.

Since the vent openers are expensive, you may want to take some simple precautions to prevent a storm from breaking them. (this has happened to us twice now)

Strong winds can force the vents open and break them. To counteract this, you can disable the vent openers by detaching them on one end. Then use a latch or a rock or whatever to hold your frame down in the gusty winds. After losing two vent openers, we now take a few moments to disable them when a storm arrives.

Your plan sounds great and I bet you will see a nice harvest from your cold frame. Oh and if you want more design inspiration, Fine Gardening magazine had an article about how to build a simple cold frame - it was a 2 or 3 issues ago.

There was an article called "An Easy-to-Build Cold Frame" in their November/December issue. I'll see if the library has a copy. The magazine has posted a video about the project.

There are some nice ideas in soilent_green's cold frame project and in many of dan_staley's posts. I think I'll do some more reading before heading out to the lumber store.

I took some big pallets that some steel came on when we were building our last greenhouse, installed some scrap polycarb on top and made a frame on the bottom of the pallet out of 2x4's and runners from another pallet to give it a little more room above the ground. I put that over part of a row in the garden, and had some great early/late lettuce and spinach. I had a rather elaborate venting system. When I saw the sun was shining, I put a couple of bricks under the long side to prop it up.