First the downsides: The larger dimension wood comes in 12'-13' lengths and is both heavy and bulky to work with. This makes it difficult, but not impossible, for one person to build. Plus, unless you have a large saw, you will need to make multiple cuts on each board. A chop saw set on saw horses is the best way to cut, but a regular hand held circular saw will also work. Measure, mark, cut and rotate. Measure, mark, cut and rotate. Measure, mark, cut and rotate. Measure, mark, cut and be done! Depending on your saw it may take up to 4 cuts. And since most saw blades are 4" or 4.5" they will not cut all the way through the board, leaving a center nub that needs to be cut. A hand saw works great for this.
Upsides: Each board is cut out of a single tree and is therefore mostly heartwood. Heartwood is the most rot resistant part of the cedar, so these boards will last longer than 2"x6" or 2"x8" boards. In fact, Alaska Yellow Cedar 5"x5" boards will last as along as treated timbers (not creosote), with University trials having the cedar lasting 25 years in ground contact! Plus the larger dimension will not bow or bend so you can build longer beds. They are easy to fasten together with Timberloc brand screws (available at Home Depot). Put in two screws per long section to fasten is to the board below, one per each short section and one at each corner horizontally. Perhaps the best part of using the Alaska Yellow cedar is that these trees used to be perceived as waste trees. Smaller trees used to be discarded when timber companies clear cut a forest, now they look for ways to use this wood instead of burning, pulping or chipping.
To construct, cut to length (4'x8' is a good size for being able to reach into the middle from both sides and it uses all of a 12' board), drill the bottom boards with a 3/8" auger bit at a rate of two holes per 8' length (no need to drill the 4' boards), level the area you are going to place the bed, put down a base of 2"-3" of crushed gravel tamped firm, screw together the 4 boards that make up the bottom course, place the competed wood rectangle on the crushed gravel, check to be sure the bed is level, drive a 3/8" two-foot long rebar into each of the four holes on the long board with a sledge hammer, re-level the bed (you might have to pry a low side up and pack with gravel to level), screw on the second and third courses and fill with soil and compost.
You can buy the boards at Issaquah Cedar.
Rebar & Timberloc brand screws can be purchsed at Home Depot.
For soil, I suggest Sayer's Fuel in Seattle. Use their topsoil (3 parts) mixed with Cedar Grove Compost (1 part).
Here are a couple photos. I will post some more later this summer when they are full of vegetables!
