I found something on YouTube where he made a frame and has put 1/4 inch wire mesh over the frame which he put inside the plastic bin, made a small cut out on the bottom of the bin to have the poop pan out in.I really like your brooder set up. I have 3 plastic bins and a huge mess at 3 weeks. I’ve been trying to figure out an “intermediate” cage set up because they are too small for their final cages.
Here's the link :
Hope it helps