Originally Posted by BanjoPicker
How thick do the rivets need to be to hold things securely in place?
Some questions don't have one single answer. Rivets are like that. There are several styles, diameters, and lengths, and you haven't indicates the thickness you're putting the rivet through, or the diameter of the drilled hole you're putting the rivet into.
So, in an attempt to answer your question, I am providing the following link, that may help you decide: