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Chris,
If you add plywood to the top of the seat frames you likely should provide a couple of vent holes. There's a good reason why the original cardboard beneath the springs had vent holes in it. When you sit on the springs, you compress the air within the cushion and without vent holes undue stress is placed on the material on the sides of the finished seats and to where that material is attached to the bottom of the wood frame leading to premature failure of the material and/or its attachment.