I'm owned by two cats who track litter everywhere and I was desperate to contain it somehow. The ready-made covers you see online cost upwards of $100+ and I wasn't about to shell out the $$$ for 2 of those... So I found two unwanted cabinets and repurposed them to fit my needs.
The first cabinet is a small TV cabinet with glass doors that I found on Craigslist for $15. I removed the inside shelf, covered the doors with scrapbook paper, and cut out an access hole in the back (which was cardboard). The litter box was a tight fit but it works, plus the cats like to sit on top of the cabinet to look out the windows (bonus!). And it's on wheels which means it's easy to move to sweep the few stray pieces of litter that make their way out... So it's not a perfect solution but it does catch about 95% of the litter that used to come out!
One of these days I'll get around to painting the oak laminate, but for now, I'm happy.
The second cabinet from a thrift store (I think I paid $60 or $70 for it) is quite a bit bigger but fits perfectly in a small alcove next to the stairs. It was also a TV cabinet and it's solid wood. Again, I removed the bottom shelf and cut out a hole in the back (not cardboard but composite wood this time). I want to paint this one too but the finish is in very good shape so that'll wait for now. The upper part of the cabinet stores extra litter, garbage bags, etc.
So for less than $100 I now have two hidden boxes for the cats -- seems like a good deal to me! Let me know what you think.