A few moths ago, I refinished some bedside tables in exchange for a twin bedroom set and a couple of tables. (I got a bargain.) The bedroom set is 100 or so years old, so I was hesitant to materially alter the head/foot boards in the event someone was interested in using these as twin beds in the future. But, I was able to fashion these head/footboards into a daybed!
The idea was to use the two footboards for the sides of the daybed. And, the headboards are for the back. The side rails, already a perfect length, hold the front and back together.
The two headboards are attached from the back, through a board and into the side rail with a few screws. And, the left and right sides of the headboards are attached to the footboards (or, sides of the daybed) with this piece below. The headboards are about an half inch too wide on each side but, not noticeable.
We added slats between the rails to hold the boxspring/mattress.
The daybed allows for so much more room than the queen bed that was there previously. The mattress and boxspring are new!