I have been wanting to redo my raised beds, and go with a all above ground type. What has held me up was the cost of materials, mainly what ever I used to construct the walls of the box with. I wanted to use recycled plastic boards, but at over a dollar a board foot, it got expensive in a hurry. I wanted to stay away from wood, rots away too fast, and I would be redoing the beds in five years. Then today at work, I get told to cut up some large blocks of high density closed cell rigid foam so the garbage man will take it away. The company I work for makes aircraft grade foam, and this stuff did not meet the high quality standards they have for their product. This is not the standard foam like what you think of foam being. It is more like a solid hard plastic, the foam bubbles are microscopic. I took home 8 sheets that I cut of 1.4"x29"x99" foam, each sheet weighs 90+lbs! This particular foam is designed for extremely high crush strength. I also brought home 4 sheets of 1.5"x30"x80" foam that is a slightly lighter density. Properly designed, this is going to make fantastic raised bed boxes. 
Now for the best part, if you did not catch it. I was PAID to custom cut this stuff to the thicknees I wanted, then was told I could take it home. It saved the company money they would have spent having it carted away!
Proof of life photo:
Now for the best part, if you did not catch it. I was PAID to custom cut this stuff to the thicknees I wanted, then was told I could take it home. It saved the company money they would have spent having it carted away!
Proof of life photo:
