The bed is double-wall steel with a factory plastic liner. Can't get to the back side. I used a big sheet metal screw, bedded in silicone for rust prevention. It's been working great for months, including a lot of washboard roads.
You don't need tremendous holding power in this application. Your tools weigh what, five pounds? And you've got two clamps spaced wide apart so there's no levering action.
The Quickfist clamps come with a sheet metal screw and they state a very acceptable holding power in their specs. I'm really happy with the installation.