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ROKTOY has a nice set up using a 3x3m tarp, one pole (pole optional if you can use a tree branch) and a few bits of string. I'm sure there is a you tube clip somewhere

As will has said, a tent will give you more warmth than a bivy bag,though both give more than a hammock

Got a wee Kathmandu that works well can't recall name. Depends what and where you go but I'd opt for a tent. If inspiring down you can stay in bed comfortably, bivies suck in the rain if your in it...