Pallets work fine, but if it is going to be a project due to logistics, use cedar shakes. Ahvatsa and Davatsa made this practice popular. Push them into the pond bottom at an acute angle, as near to flat as possible but maintaining enough exposed surface on the bottom of each shingle (so the FH have a place to stick the eggs). Do this in 6 - 12" of water at random spots around the shoreline that is accessible.