Those plastic liners are junk.
1- they're slippery , nothing stays in one spot.
2-they vibrate against the bottom of the box and wear the paint off , causing the box the rust out.
3- I've seen many of them along the interstate that have blown out of the bed , ripping through the screws that were supposed to hold them in.
Spend the $$$ and get the box Line-X 'ed