I'd need 3 boards (6 inches thick each) to get 18 inches under the rails, and 4 to get 24 inches.
They'd have to slip forward 9inches or sideways 15 inches under weight, to fail.
With the weight of the vechile on them, they are not going to go anywhere, in fact you'd have a hard time moving them with a sledge hammer..
I feel more confident with these holding up the vehicle than a jackstand.