Cat keep peeing/pooping over the side of a litterbox? Use a bin!
Using a tub prevents the cat from urinating or defecating over the sides and reduces the amount of litter scratched over the sides. The only place it can be scratched is out the door. I offset the door because I felt there was less chance of the litter being scratched out if it was on one end rather than the middle. I used a Sterilite 19 Gal./72 L modular stacker tub purchased at Wal-Mart. 253/4" x 183/8" x 131/4" If you have more than one cat you might want a wider/longer box, and add more litter. Cut the "door" with a razor blade. Six inches from the bottom of the bin to the bottom of the door worked for my cat. Adjust for your cat. I added whatever litter was in the old litter box plus a 25lbs box of litter, it was three inches high on the inside. It could probably hold another 25lbs box without reaching the bottom of the door. A 25lbs box (three inches) is deep enough the cat isn't scraping the bottom of this litter box and the width of the tub gives the cat plenty of room.