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Most cold-frame construction is sunken in the ground 6-8" to protect from wind and utilize ground warmth. Glass windows on hinges are often employed to keep heat in on bitterly cold-days, and then propped open on warmer days to let heat escape. They usually face south toward the sun.
Cold frames are much more common for vegetable production than landscape plant production. Cool season crops like spinach can be grown and harvested well into winter with a cold-frame, and slow growing warm season crops like leeks can be started well-before the last frost date in spring, to get a jump on the growing season.
If you are looking to force landscape plants along, I would go with a poly-house. They come in many sizes, are cheap but durable, and will both protect against freeze-kill in the winter and hasten things along in spring. If you use the cheap 1 year 4 mil poly, you can simply slice slits in the poly as things warm up to let heat escape, while still retaining protection against late freezes. I don't know where you are nate78, but Midwest Trading in St. Charles IL. has a catalog that is an excellent resource for poly houses.
A concrete slab might actually retain cold longer than regular ground, and slow the emergence of your plants.
Good Luck!
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