Not drilled into my Ramboxes, but by looking/feeling through the drain holes, it just feels like 1/8" - 3/16" thick plastic.
The Mopar riflemounts are expensive and don't have the locking system NWSDE is looking at.
NWSDE: If you want to use the Rambox with a hardened, locking mount, you'll have to drill. However, have you read about the folding rear seat modification?? (Put a stack [~3/4"] of washers under your rear seat mounts, and you can fold the rear seat backs and access a rifle-shaped space which most people won't know how to access.) A TuffLoc mount happens to fit right there, and if you get the electric lock option, you can hide the button by the driver's seat. The 3-second delay is just enough to get out, open the back door, tilt the seat, and grab the rifle...