Saw it up
Screw some legs together
Screw a frame together
Scavange some bed slats from an old futon
Replace caravan cushion mattress
(There’s also a wonderfully comfy IKEA mattress topper under there, by the way. And an electric blanket.)
It was a pain to make the frame, because the whole thing only fits in the van one way, making screwing it together really awkward. So no, it’s not square, but it is stable and that is good enough for me! The method I used is from Instructables $15 Bed Frame.