You can replace the traditional box spring with a plywood board—a piece of plywood can offer the consistent support a foam mattress needs, unlike a box spring. However, a plywood board does have its drawbacks. It can make it impossible for air to flow through the bottom of your mattress, allowing heat and moisture to collect and promote mold growth. Look for a plywood board with a moisture-resistant coating and consider a waterproof mattress protector to keep out moisture.