If you prefer the look of it, I can now make your frame with mitered corners, meaning that the ends of the rails are cut at an angle, like on a picture frame. On fir frames, this means that the end grain of the long rails will be hidden. Hardwood frames will not have or need the end caps.
As an additional bonus, bed frames with mitered corners also have a pair of floating tenons connecting each corner together, further strengthening the frame.
If you like the original design with the short rails butted against the long ones, that’s fine and you can have your frame made that way.