Take @Timmy's advice and use HD aluminum foil. I used up the liners that came with my IW and went to foil and never looked back. Stuff works great and you can get it to form to the tray much better so the grease flows where it's supposed to go.
Update: Liners delivered OK. I have been using foil, but I have been seeing that the foil "billows" up, as though the fan is blowing air between the drip tray and the foil. I got new liners so I can use foil and mold it around the liner's edges, eliminating the "billowing." The liners should last indefinitely, covered in foil.
Yeah, I know this is pretty "meta," using foil to protect the foil liners...
I don't know what the differences are (if any) between the Costco models and say an Ironwood but I've been using HD foil on all my cooks and never seen the foil billowing (like a flag?). When I apply it I use enough to fold it generously over all four sides and it stays put.