I used this kit from Deflecta-Shield
Drilled a 2" hole in my over the rail bed liner, stuck the J clamp into the hole, drilled a small 1/2 hole down through the bottom of your toolbox to line up with the J clamp. Stick the 4-5" bolt they give you up through the J clamp into the toolbox and nut it inside the toolbox. Works great and you dont have to drill into your bed rail.
Make sure you have at least a 1/2" between the toolbox and bedrail for your bolt to fit up through. Mine was a tight fit with a Contico Tuffbox.
You can buy this kit at Advance Auto or Autozone for $10. I found mine at Advance.