I knew before hand I would have this problem. That the bed would be a pool of water that’s why I used my straps as an “X” across my bed to hold up a plastic sheet. Didn’t want to use wood cause I would need to paint it to prevent it from getting water damage. But the plastic I was using was too flimsy and the the straps of the “X” was too big of a gap that it would sag. So I just got got more bed cleats and added more straps to close up the gaps and it worked nicely.