We got one of those blue closed cell camp pads from Walmart. We cut it the size of the cushions and then put them under the cushions. It was a big improvement, but we’re still looking for denser foam to make new cushions with.
I tried replacing the cushion foam with memory foam topper material, but it provided less support than the original foam. Doh!
I’ll be interested to see if you find a source for dense foam to re-stuff the cushions with. We looked at the craft stores, but the stuff they had was pretty expensive.