I am reminded of the fifties and sixties when some of us on a budget used to put 6x9 speakers, the ones with the biggest magnets you could afford, up on the rear deck to get better sound. Now the smart way to install them was of course in the deck facing up, not sitting loose on the back deck.
One slam on the brakes and too many kids got head injuries from flying Jensens and Audiovoxes becoming genuine high speed missiles. Rarely would the twisted on speaker wires provide much resistance to forward motion. One of the strange bits of knowledge I picked up in the insurance industry for many years.
If something is going to go on the back seat deck, it ought to be a box of Kleenex...
Those of us with ROPS and seat belts have to keep this in mind, what will go flying around, or come loose, if we tumble? I had thought of using a large collection of Universal Gym weights I have, the flat kind that are 15 pounds of perfect iron that stack marvelously. And after reading this, I will make sure I build some kind of very strong retaining strap or top on it for sure. I think those weights would hurt more than wet sand, and I don't think either of us want to try.