Flatbed....rails? boards? keeping the camper centered...

I have tool boxes mounted on the long sides of my flatbed. This helps to line the camper up because it's kind of like having bed sides. However, the camper doesn't touch the tool boxes, that is, they don't help keep it from moving. The 3/4" rubber horse stall mat it sits on and the tie downs keep it from moving. It moves very little at all.