Mid century modern king bed is 76 inches long while a queen size bed is 60 inches long. Therefore, a queen headquarters is approx. 16 inches narrower than a king headquarters. Nevertheless, with creativity and little time at home improvement center, you can do the queen headquarters work with a king-size bed. In fact, most modern steel bed frames can accommodate both queen-size and king-size beds. And the headboard attaches to the bed frame, making the headboard extension much easier.
Remove the headboard from the mid century modern king bed. Measure the width of the headboard and compare the width of your king-size bed frame. The difference in width should be compensated by using wooden posts used at the ends of the headboard or intermittent along the face of the board. Each headboard is different. Determine how you want to apply the positions that make up the difference in width. Cut posts to a desired length that complements the height of the existing queen bed. You should be able to use two 6-by-8s or 8-by-8s at both ends and five 4-by-4s, evenly distributed over the headboard. Sand the positions that match the finish of your existing queen headquarters. If you want to expand the headboard with 4-by-4s, cut the headboard into 4 equal widths.
Drill three pilot holes in each of the positions and ends of the head yard, and ensure that these pilot holes line up between the headboard and abutted posts. With the post and adjacent headboard, screw and lower the French screws or drill suitable holes with a diameter for the cam fastening system. If you use cams, install and tighten your cams. Add bumps or caps to the ends of the posts, and paint or flatten headboards to create a uniform color and finish for the piece. When the finish is dried, attach the mid century modern king bed frame to the headboard.