I'm having a hard time visualizing "plastic spring style cushioning", but I would try to find a way to make industrial strength adhesive Velcro strips work. You could attach a piece of rigid flexible plastic to the springs with zip ties or wire and then adhere the velcro strips to that. I use this velcro all the time. The trick to get it to adhere really well is to first apply rubber cement to the substrate and let it dry completely. Then apply the adhesive strip onto the cement. This makes for a very secure bond. For the cushion, you can use this same method with the rubber cement and velcro as it remains flexible. I would use masking tape when spreading the cement to avoid making a mess on the cushion.
