Old tee shirts. Hang several so that they hang like drapery. Make sure the bottom edge does not drag the floor of the catch box. They need to swing free so they will absorb the energy and drop the ball straight down.
A while back, I read that cheap pillow cases from Walmart, when hung in your catch box, are a good solution to bounce-backs. The material seems to be just the right 'weight' for the job. I've been using them successfully ever since. I hang two of them. I prefer the 'black' color . . . makes my targets show up much better.