Shower pan liner. It's in the plumbing isle in home depot. It's a 20 mil poly very thick plastic. It works great. Or you can also use pond liner it's a little cheaper but thinner mil. They both work great.
I would suggest pond liner personally, shower liner is much thicker, but it's also pretty expensive. I got my pond liner for $4 a foot in a 5' wide roll. I saw at home depot shower liner was double that price. Both will do the same, and have great durability
you can either staple it at the top and corners, or you can just put it in there and forget it or use an adhesive glue and adhere it to the bottom and up the sides a bit.