You could just hang something the correct size behind your bed. I've used old doors, stained glass, iron or fencing or gates, quilts etc.
Tina...I have used a really large picture then pushed the bed under it and it looks fine.
All kinds here
What style do you have in mind?
lots of ideas here: https://www.hometalk.com/search/posts?filter=headboard
Here's one: https://www.hometalk.com/40190183/diy-headboard-that-s-easy-to-build-and-install-no-holes-required