I can't find my post on this modification others have done.
I put mine exactly where you're talking.
On the top side, I drilled 1/2" clearance holes, then I was able to put a screw gun driver through those holes and used self tapping #12 screws to attach into the frame.
I had to use a little plumbers putty to stick the screw heads to the socket until I got some pressure on them.
2016 Toyota Tundra SR5, 5.7 V8