Two shoe box lids, one smaller than the other. A piece of card cut to size for the headboard.
Stick the smaller lid right sides together with the bigger lid, the smaller lid becomes the base. Cut the piece of card so that it fits snugly into one end of the bigger lid and the height you want for the headboard.
This is what you end up with...
I think that for the small amount of work it took this bed is pretty good, if I do say so myself. Even the bedding didn't take long, it was mostly just straight sewing and the mattress is just some wadding sewn into a cover. The crochet blanket was very quick and easy, I did make it smaller than in the pattern so that it would fit the bed. And I made quite a few mistakes because I was excited about getting the bed finished!