"er78mph"....."RoyB" has the best idea and cleanest setup with sufficient support for batteries. Go to Home Depot or Lowes and buy enough 1" angle to span across your tongue. Rather than drill 1/4" holes, buy four (4) large self tapping screws and screw both pieces of angle iron down at each end. Use some smaller self tapping screws with large fender washers to secure the battery boxes to the rails.
Once done, spray the steel and screws with some gloss black paint and you'll have a nice battery rack. You can also buy some aluminum diamond plate at HD and Lowes and cut a piece that fits between the rails and make it look really nice by trimming it at an angle that matches the "A" frame. You can rivet it on or use the self tapping screws.
If you want to get fancy, you can buy some plastic coated cable at HD or Lowes and make some cable locks.