As just mentioned, do first determine why you need/want to raise your bed.
So I built my first raised bed in a north/south orientation in a south-facing backyard in Toronto, plant hardiness zone 6a/b depending on circumstances. I dug down three feet, buried a couple of large manitoba maple saplings (about 6" diameter at breast) and dropped two years of
compost, a yard of composted organic cow manure, some seaweed and shellfish
compost, and the soil that had previously filled the hole I had dug. I surrounded the resultant pile with a
fence I made out of 2' x 3.5'
pallets. My intention was for the enclosure to slant towards the top slightly, allowing my alpine strawberry guild to grow in the spaces between the slats, which actually worked well, for its own part.
Where it fell down was this: I had designed and planted a minimal to no-water bed because I knew that I would be keeping
watering to a minimum. When the dry part of the summer hit, because the soil spaces between the slats allowed the air to dry out the whole bed, I had to
water it, which I didn't want to do.
So first determine if you really want it raised. If you do, I would think about its permeability. I like the planned deterioration that is represented by fresh cut hardwood that you then innoculate with
mushroom spore. If that's not your cup of tea, I would think about something with natural antifungal properties like the aforementioned cedar, cypresses, or
black locust.
I have a potter friend, and have been toying with the idea of simply mulching with large pieces of her unglazed clay pottery, with larger pieces rough cut and assembled like you would a mortarless masonry wall, all fitted together to contain a hugelbeet. This, though, would be less about containing the pile itself and more about containing the moisture. I'm sure it would also look fantastic, like a garden gnome massacre.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein