I made my raised beds to allow use of old glass storm doors as a kind of cold frame top. It's great in that the doors just lay on top of the raised bed frames and don't cramp plants, but not so good because they are a little heavy to move on and off. I can usually scoot them off in increments, but sometimes need help. I already had the old glass storm doors so thought I might as well use them. They are great in early spring and late fall.

I am not familiar with the venting units for coldframe tops, but just don't have the budget to buy a lot of extras. If you have info on them, post a link here - maybe they would be helpful for others.