Here's the old door I found:
It's exactly the right width for a king sized bed. All I needed to do was add legs! I still can't get at my mitre saw, it's buried by plumbing supplies and beadboard panelling - sigh! I wanted to impress everyone (and myself!) with some gorgeous trim details... but I'm too impatient and didn't want to wait until next summer to work on my headboard so I used my circular saw to cut a piece of 1x4 and my nail gun to attach it to the long side of the door (hinge side).
I measured from the floor to where I wanted the top of my headboard to be and cut my 2x4s to that length. Then all I had to do was use screws to attach the 2x4s to the back of the door. That still looked a little unfinished, so I added 1x4s to either side leaving a a half inch reveal to cover up the joint.
I primed and painted the door and drilled holes in the legs so I could attach the headboard directly to my bedframe.
I used nuts & bolts (the kind for attaching stair boards to metal pre-fab risers) to attach the headboard to my metal bed frame. I used four of them total to hold the headboard in place - a nut with a 2 1/2" shaft, two washers and a bolt for each fastener. You could also attach the headboard to the wall either with screws or using a French cleat, click HERE for the how-to.
Another project down! several hundred to go lol! To see my finished Master bedroom post, click HERE