The Book of Buckskinning II has pictures/plans for making a Rev War folding camp cot - pages 199-202. It was based on pictures of an original. It's made like the folding canvas camp stools. But the full length does not fold.
George Washington is also supposed to have had a set of canvas stools that would be tied together for use as a cot - with a "matress" of some sort laid on top of them. This would give you more options for use and folding/packing.
A number of people have made and used a version of a wood/canvas hospital stretcher. The more modern ones need to be covered up to hide the new parts. But even the old style ones have one major problem for many people - they are too narrow to sleep on. The side rails hit me right at the arm/shoulder joint. A friend used them for his kids when they were little - one at each end. Worked great for them.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands