lining for raised bed

So this is my first time making a raised bed and I am not sure how to finish it off. The wood I have used is heat treated, and the bed is slightly raised with a wooden bottom(I don’t have a big garden with soil so I cant really just put it on top of the floor). I have some “extra strong” outdoor cover sheets, and I dont know if it is a good idea to use it to line the bed to prevent rot. I imagine I would have to drill some hole at the bottom for preventing waterlog. But I don’t know, should I leave the wood bare, or use the cover, or use a completely different material altogether? I am on a tight budget and not sure what I could use to extend the lifespan of my raised bed as much as possible.