We recently upgraded to a king sized bed. It's the Sultan Finnvick from Ikea if you're curious (I wanted the natural latex mattress but we couldn't justify the 700$ difference). We looked around for a headboard/bed and didn't find many king sized options for a reasonable price, so we decided to make a headboard. I was going to go the usual plywood route until Mike found that a 36x80 inch hollow core door was thicker, lighter, and the perfect length for a king sized headboard.
We cut 3 yards of 1/4 batting into 3 sheets and covered the door, stapling each layer as we went. I recommend using a pneumatic stapler over a staple gun as we need the extra power to get through all the layers.
After we attached the batting we covered the door in three yards of natural linen, scored for 40% off at Joann Fabrics. I liked how the staples almost look like tufting. We stapled the shit out of this thing to make sure it stayed tight.
We mounted the headboard on the wall with a French Cleat for 15$