Looking for idea on how to support my camper of truck?

Thanks, due for a coat of paint, never know, it could rain again :). The Xs are the simplest & effective. While another full sheet on top would work seems might be a pain to store and use, though could be thinner & cut? I looked around there are a few threads in the TCU, But I saw one member, Budster, using 2x4s across the top of the Xs. Not sure, maybe notched also so they sat flush too along with edge of ply, especially across the front, more contact and more structure to X?