Sub box in 1G problems?

[ShastaBlasta14]

Ive been planning on buying a sub box for 2 12"s, but as I was cleaning out my trunk I realized there might be a problem: The stuff that covers the spare is just cardboard. I doubt that the cardboard will support the weight of the box very well. How have you guys gotten around this problem? Could I make a new cover out of 1/2" plywood

Thanks

 

[Drummer1503]

dot worry abot the board i have 4 12's in my trunk and the weight of the box hassent done anything to it yet