Upcycle used plastic shower curtains, children's (leaking) wading pools, or black garbage bags by using them to line 1/2 whiskey barrels for container gardening. The plastic will help slow the rotting of the wood plus hold moisture in the soil for plants.
Duct tape around the outside edge will hold the liner in place. Or you can try staples.
Don't forget to poke a couple holes in the bottom of the liner for drainage!