I used the one that cost $28 and it worked just fine with a block of hardwood for a backing.

Pretty sure I used http://www.marcospecialties.com/pinball-parts/07-6688-18N on playfield plastics, but the length of the rivet is going to depend on the thickness of both the plastic and the bracket; rivets are fairly inexpensive and you may want to just buy several different lengths and do a little experimenting before you commit to your playfield plastic. Word of caution...if your rivet is too short, it won't seat properly against the plastic since the setting tool will bottom out in the rivet too early; this results in a loose plastic and you will have a hard time drilling out the rivet since it will just spin in the hole.