A buddy of mine put a sheet of semi-rigid plastic about the thickness of a good sturdy city trash can (one of those square types with the wheels and attached lid - see link below) between the burlap and the dense foam. His theory was that it would spread the weight across more springs. Worked out really well in my opinion.
[edit] ...the type of plastic like they use on these trash cans -
http://www.harvestofdailylife.com/wp-content/uploads/2006/04/city-trash-can.jpg
