I found the simplest answer was 4 inch breeze blocks. You can lay them on side or end if you want, but I just laid them flat, 3 high. If you want somewhere to sit, a couple extra on top do the job. They build houses out of them, so they should last for a good while. They don't have to be cemented in place, which is quite useful if you want to change its shape. The top of the wall is quite useful for growing in pots. You can also increase the height as you grow older. Depends on how much you want to lay out. £50 will make a 5 sq.m bed. 33 blocks for each level. 9 inch hollow blocks could be incorporated which would give some growing space in the wall.