We have a good old sofa, purchased second-hand about twelve years ago.

It's a terrific napping sofa, the kind you just don't want to get up from. Anyway:

after any amount of mere sitting, the cushions pull away from the back of the sofa. It's a pain to be constantly tucking the back of the cushions under the the backrest cushions (can't think of a more elegant way to decscribe this tedious routine.)

What can you suggest to put between the back and the sofa-cushion back to keep them in place? I am NOT skilled at sewing. Are there any cheap, ready-made products out there for this problem?

Thanks!