Go to autozone or napa and get interior plastic retaining nuts! The guy behind the counter will look at you like your a retard so don't both to ask! You will have to look in the interior parts section, they were used in a lot of GM vehicles in the 80's to secure trim pieces. Thats what I used and works great. I think I got the 1/2 ones, can't recall though, but they are self threading so when you screw them on, they will lock them selves on.
They are black
they are hex shaped and they are thin, like a washer but are pressed to look like half of a nut.
And they are a b#tch to explain!