Put a piece of plywood between the cushion and spring frame.
So most people here seem to be of the opinion that the sofa beds are complete crap...
Can I just take a moment to express an opposite opinion (for any prospective buyers on the fence)?
I can't speak to the fancy leather one, but we got the cheapest princess version with just grey cloth. Bought from shop4seats, paid $589 + $200 shipping, so still not ultra cheap. We sleep on it every single night, and lounge on it nearly every day. (We live in our van) It is quite nice! In fact when we get the opportunity to sleep in a 'real' bed, we often turn it down because the sofa bed is more comfortable to us.
I like the minimal design of the legs/anchors because it leaves a lot of room underneath for gear storage. It doesn't have seat belts, but if we do add them, the belts have to mount to the floor anyway, so I don't understand the ' bad in an accident' concern. It's not designed to have seat belts fastened to it...
It's not really bouncy. We have suburban Springs for lift and higher payload, but also have a few hundred pounds of gear and stuff inside, plus a Tw200 motorcycle on the hitch sometimes.
Had the sofa for over two years now.
Tl;dr we love our princess sofa bed,very comfortable, no regrets. But I can't speak to the more fancy models.