Installing THULE 822 XTR bed rider rack ??

Howdy folks...

...love the rack as I've had a few over the years...my pickle is that with the factory installed bed liner, when mounting the rack across the bed there is a "sloppiness" created with the gap between the bed liner, the bed and the bike rack...
...Thule has "quietly" put 2 holes on the bike rack mount that would allow me to drill into the side of the bed (DON'T WE ALL HATE THAT) and secure the bike rack with a snug fit...
...my real question is what type of screw-lag-??? do, I use and should I "glop" some silicone or tar in there after the hole is drill to curtail on any rust ???
...thanks for your time...