What to put under thin sofa cushions?

[clangermum]

The sofa cushions have got thinner over time and I'm wondering if I can put anything underneath them, like the foam you can put under mattresses? Has anybody done this, or used anything else?

Do the cushions have a zip closure? If so, could you put foam inside? Might be better than simply underneath where it could shift about, and perhaps be visible.

Good point early - will try this