The foam you use for your DIY bench cushion is mostly a matter of personal preference. Most upholstery foam comes in low, medium, and high densities, but contrary to popular belief, the density doesn't actually affect how "cushy" the foam is; it's more about the durability of the foam. All three feel about the same when you sit on them, but low-density foam will "go flat" and lose its spring a lot faster than high-density foam (think like a year of daily use vs. over 10 years of daily use.) But, of course, high-density foam is more expensive than low or medium density foam. I prefer to use medium density foam because I find it strikes the right balance between being a quality foam and not being outrageously expensive. If you're on a really tight budget and don't mind the durability trade-off, I've heard of people using low-density foam mattress toppers or egg crate foam inside their cushions.