Nancy, a solid hardwood seat frame to attach and support the springs and no less than 2 silent tie-wires running across and clipped to each spring.
I recommend a foam core over a coil spring unit. Either a Ultra-cell or HR foam with no less than 4" thick and 1.8 lb density, the higher the density the longer it will last, but the costs of the cushion goes up substantially as well. The density has nothing to do with how soft or firm it sits, thats in the next number= the ULD or ILD. A medium soft is around 22 - 24 ULD, a medium is around 26 - 28 ULD and a medium firm is 30 - 32 ULD. The manufacturer should be able to provide those specs. This foam core should have also a soft foam wrap (topper) over the core to add comfort/soft touch and fill out the cover. This soft wrap should be no less than 1" and be also the same Ultra-cell or HR foam, just a softer ULD around 18 ULD. If you want a cushion that will last and hold its shape, then avoid any polyester wrap or feather envelope. The reason I don't recommend a coil spring unit is not the spring unit that don't last its how it constructed as the wrap around the unit if not done correctly will break down in a short time and will loose its shape and support.
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