1 plate is ok, 2-3 maybe if they're rubberized. However, this
won't get you past 5-10deg incline. Any more plates you put there
is gonna be dangerous. Just imagine if they slide off while you're
in the middle of your bench...and if they're solid metal ones, it
isn't hard to imagine them sliding off.
Besides, where will you sit your butt on this bench? There's no
seat!