Go to a hardware store, and look for jack-nuts. Or maybe they are called jack-screws.
These are things with about 1/2" between the nut and the outer face of the jack-nut. You insert it through a drilled hole in sheet metal and when you tighten a bolt in them, the nut pulls up tight against the inside of the sheet metal and you get 4 parts that stick out against the inside of the sheet metal.
I try to coat the drilled hole with some sort of anti-rust paint before putting the jack-nut in, so you don't end up with rust a few years down the road. I use stainless bolts, so rusty bolts aren't an issue, and you can get them loose years later. You want to make sure they are tight against the sheet metal when you are installing them, so they don't turn in the door. I've sometimes had to use tiny long nosed vise-grips to hold the outer flange on the outside of the door to get them really tight.
This is what I've used to mount west coast mirror braces on my doors.
All that being said, I've been tempted to remove the west coast mirrors from my '97 and install
flip up tow mirrors like I have on my '96. I really like them better, they are power mirrors that are heated for winter use, and there's probably a lot less air drag when running empty. Remember, every little bit helps for fuel mileage!