Here's a link to those cheap-o Walmart food containers.
https://www.walmart.com/ip/Mainstays...Prep/938593899
The top is clear, so you cut a hole in the side (again sand away any jagged edges) and place it in the enclosure upside-down.
That link is for 5 containers for $2.95. It doesn't get cheaper than that. They make for great humidity boxes also.
Plus, at that price, if (when) the snake poops in them, easy clean up! Just toss the container and make another one!