The No Sag or also zigzag or serpentine springs are widely used in chairs and benches. For example, think of Ikea furniture. You can buy these springs in different thicknesses and lengths. And even on a roll. The latter is useful if you use a lot of it. You do need special tools to cut the hard steel springs and bend the end so that the spring catches behind the metal clamp.
For example, this Osborne end former. I always use the metal clamps and not the plastic clamps, they break down way too quickly.
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