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A hammock.

More specifically, an 11’ or 12’ gathered end (aka “Brazilian style”) hammock made from a solid fabric. Not a typical spreader bar hammock you find in a suburban backyard, and not one made of a rope net.

I learned about them from backpacking as an alternative to a tent. There are some advantages and disadvantages in that context. There are some really nice ones with built in bug nets and pockets for gear you might need at night and shelves to store things and all kinds of cool little extras. There are also cheap ones on Amazon that are basically just a rectangle of nylon for $30.

For sleeping, you want it to sag quite a bit. 30° is a good starting point, and where I leave mine. If the ends are suspended apart from one another by 83% of the hammocks stretched length, that’s pretty damn close to 30°. It also helps to hand the foot end slightly higher, other wise you’ll slide down toward the center.

>but I don’t sleep on my back
In a large hammock, hung with a deep sag, you can lie diagonally (not with your head at one tied end and feet at the other) and sleep on your side. It’s hard to explain

Pic is one of my camping setups.