@TheyCallMeDave You could allways get 2 wood planks,fix them together using old straps,1 plank on top other one hangs on the side. Do that front and back at the side,so 4 off those. Then strap . Should keep everything more in it's place.
After seeing that pic, I must admit it looks interesting as far as securing the load goes on those trees.
Yeah, they start with two bales of hay on the back of the trailer that you secure with a strap, then start leaning trees against it and working forward. Quite a team effort.
Sod is so uneven and squishy v boards like that would not work very well. It is not going anywhere once the tarp is on.