I also built myself a bed ( double ) and I still have it. I sleep in it every night. It is very much like your bed except the posts are shorter and therefore they do not form a frame at the top.
My bed has 3 hardwood rails that cross the width of the bed and this is what supports the box frame. My center cross rail is dovetailed so that the side rails can not push out.
There is a book of a couple in the US that built everything on their farm. The couple was Helen and Scott Nearing. I never did read the book except I glanced at it. It was stated that as Scott was drawing his last breaths he lay under the roof of the house he built and in the bed he had made.
It gives you alot of satisfaction when you take rough lumber and produce a piece of furniture.
Craig Clayton