I've just bought a set of Pro Rack roof racks for my 2005 Mazda Premacy. This doesn't have gutters, and so it uses a single screw mounting point for each mount, under some rails on the roof.

This is a grub screw with a hex bolt at the top.

The rest of the mount goes together like this:

and

Three out of four of them looked like this,

and worked fine, but the last one looks like this:

It looks like water has got in there, and the whole thread is rusted away. I tried putting the bolt in hoping that there would be good metal underneath some surface rust, but the whole thing looks rotten. As far as I can tell, there is not enough good metal remaining to even tap a larger hole.

My question is:- does anyone have any ideas as to how I could get the grub screw (or a similar thread screw) mounted securely where the rusted hole was? A friend suggested filling it full of epoxy, but I am not confident in the integrity of the metal around the hole. I thought a bolt with a largish head from the inside of the car would be an option (after taking out the roof lining), with tons of silicon or other waterproofing type stuff. (And yes, I know there will be a general rust problem under that rail...)

Does anyone have any brilliant ideas?