5 Packs Cat Scratcher, 5 Packs Refill Cat Scratching Pad Cardboard for Indoor Cats and Kitten, Large Size Durable&Premium Recyclable Cardboard Cat Scratcher

I bought some cheap cat scratchers basically. I taped two cat scratchers together and covered the bottom with a flat piece of cardboard. But you can probably go even cheaper and DIY something by getting some "corrugated cardboard" and use a bit of glue or tape.