I got a 2" topper of high quality natural latex foam and the back issues I was having when using a cheap polyurethane foam topper went away almost immediately. The poly foam started crushing down thinner in the middle the first night and had much less support where I needed it (like almost none) after a mere few weeks. Some polyurethane foams will last much longer, but latex is the stuff to get if you don't mind the cost. Poor quality poly foam is a complete waste of money. Not sure how to know which polys are better. Latex can last 20+ years I'm told.
And I stuffed various thin flat items under the table area (on the table top, under all the cushions) since it seems to sit a cm or so below the seats on either side.
Actually, the stock cushions were not that bad alone, just felt those gaps a bit. But bare stock cushions is better than with a foam topper with no support anymore.
I sleep like a baby in my Scamp
since the latex topper!