Lay them on the floor/bed of the truck?
Why not pad/protect all sides and between speakers as discussed, and stand them up, speaker feet/bases on the bed (listening orientation), with ratcheting straps securing the speakers to the walls of the truck? Provide additional protection/stabilization during transport using layers of corrugated cardboard (flat storage boxes) between the speaker padding and the straps, and mattresses and/or other secured furniture to better hold in place.
It's much simpler than I made it sound.