When I have to brace a door for hinge or lockset operations, I use a door buck made from scrap materials like the one in the drawing. The base is made of 1/2-in. plywood and a couple of 2×4 offcuts. It’s important that the plywood be thin enough to bend a little under the weight of the door, which makes the 2x braces angle inward, pinching the door and holding it steady. If you’re concerned about marring the finish of a door, make the gap between the braces a little wider and line the braces with strips of carpet.
Ed Wilson, Seattle, WA