Here is what I use for one of the best and most secure grounds I can make when installing an amplifier. This is a zinc ground lug that will cut it own threads when you drill it. I like these when compared to the other self tapper screws because these ground lugs will not strip out as easily. 

Here is how you use it:
First drill a 1/4" hole in the metal you are grounding to
Use a wire brush to scrape the paint off around your hole.
Use a 1/2" nut driver to screw the ground down. This will not be easy to do by hand, it will be cutting its own threads. I use a power drill to do this. 

*In some materials that are too strong you may have to drill your pilot hole one step up from 1/4" to 5/16" and it might help it cut its threads. Some older American vehicles with strong metal may need the 5/16" pilot. 

If you would like to purchase a nice centering wire brush, the same one that I use in my installs for this method of grounding, you can find it at this link:
https://www.franksaudiomotive.com/product-page/centering-wire-brush