Your method would basically allow a large flow that would pool in a single line maybe a foot or two wide. Instead, I would drill a bunch of 1 inch or 2 inch holes all the way across the edge of the plywood, maybe 2 or 3 rows of them, and frame the plywood with 2 by 4's. Start dumping the paint onto the plywood and the frame will fill up and start dripping out of all the holes along the edge hanging over the road. This would sprinkle the paint the whole width of the truck bed! Same basic design!
Your method would basically allow a large flow that would pool in a single line maybe a foot or two wide. Instead, I would drill a bunch of 1 inch or 2 inch holes all the way across the edge of the plywood, maybe 2 or 3 rows of them, and frame the plywood with 2 by 4's. Start dumping the paint onto the plywood and the frame will fill up and start dripping out of all the holes along the edge hanging over the road. This would sprinkle the paint the whole width of the truck bed! Same basic design!