Aluminum foil or the very expensive Traeger liners last about the same and the foil stays flat. Being a cheap bast*rd I prefer the foil.
The Traeger accessories seem to be under MAP pricing, hard to get a deal unless you walk in and haggle. It takes me less than 5 minutes to pull the pan and cover with foil. Both the wide and regular foil length work fine. The narrow requires 2 pieces and takes the same amount of time, it's cheaper and easier to handle on a windy day