No picture but my parents did this. They went from a queen to a king mattress but kept the bedroom suit they purchased when first married. They wanted to keep the matchy matchy and book cases that were the headboard. Some headboards would be less obvious than theirs was. Who sees your bedroom anyway. It's your business.

Why not just go without and use layered king sized pillows and large art to create balance.

Can't imagine it would look good as the mattress will be wider than the headboard.

I cannot imagine any reason to do this so explain yourself. Yo an easily build a proper sized haedbord to mount on the wall with french cleats for the king mattress . Please give us info. Here in comments DO NOT start another post .

I can well imagine many reasons to do this. I'd hang a large piece of fabric on your wall that is the width of the mattress and use it as as a backdrop. Because it is the width of the bed, it will help distract from the headboard being smaller.

Even better if the fabric wraps around to the sides.

FleaMarketTrixie · More Info

This picture is quite quaint, but it could work with any style of room.

Add some plug in wall sconces above the bed to further distract from the size of the headboard.

https://furnishingtips.com/can-a-headboard-be-smaller-than-a-bed-we-find-out/