Maybe an army cot? If the canvas is reasonably taut, it doesn't bow too much.
Maybe a collapsible or folding table, such as
this one for instance? Though I would be somewhat worried about the sturdiness and tip resistance for lying down. The modern plastic folding tables might be a better option if you have the space to carry one; they're a whole lot lighter than the older particle board tops, and nearly indestructible. If you shop at a place selling school furniture, you can find ones with various leg lengths or with adjustable legs (for elementary school kids).
