I put something on floor at front and rear for bales to "sit" against (4x4 or built pipe rails to go on stake pockets) then strap them with one strap. All on top row get strapped , all 4" wide.  9 on the floor takes 8 40' straps for 2 wide on top and 2 30's

The Feds say 5' and over 1100# needs two on front if there is nothing in front of bales