comments:
Get the reusable washable pads, the bigger ones that are like 30"x34", and layer them on top of a twin-sized waterproof mattress protector over the couch cushions. (Actually I just noticed they make them specifically for couches now, I'll be picking a couple of those up.) The pads have a plastic underside so while you can overflow one if you try real hard, generally nothing "soaks through". I even put one of them under a slow drip under one of our sinks until a plumber could get there and it would last at least 6 hours, and kept the wood dry.
Use washables, and expect to wash them. I put on multiple layers of waterproof mattress covers like this so I can just take one off and have another one underneath. It's plenty soft to lay on but you can put a fitted sheet over it if necessary. They do make very large washable pads too, like this which is almost 60"x40" or 100cmx150cm. If you don't have a washing machine in the home, I do empathize - you really want disposable chux to work but they're just not up to the job.