Litter box smell is rarely about the litter. It's about plastic, frequency, and airflow. Here is what moves the needle, roughly in order of how much it helps.
1. Scoop every day, with a scoop that actually sifts
Nothing else on this list works if clumps sit for two days. The job gets easier with a scoop that separates clean litter from clumps in one pass instead of three; a fine stainless mesh like the Fast-Sifting Litter Scoop does that, and it doesn't hold smell the way a plastic scoop does after a few months.
2. Retire the plastic box
Plastic scratches, and scratched plastic holds odour no matter how often you wash it. That's why a box that was fine for a year suddenly isn't. Stainless steel doesn't scratch the same way, wipes clean, and doesn't absorb anything. A high-sided one like the Stainless Steel High Wall Litter Box also keeps kicked litter inside, which is half the mess.
3. Full litter change on a schedule
Topping up is not changing. Empty the box completely, wash it with unscented soap, dry it, and refill every two to four weeks depending on how many cats use it. Skip scented litter; many cats dislike it and it mostly masks rather than removes.
4. Deal with the air, not just the box
A small purifier that sits beside the box and runs when the cat visits deals with the moment smell actually happens, instead of running all day. The Litter Box Odour Purifier is motion-activated and fragrance-free, with no filter to buy. It is a complement to scooping, not a replacement for it.
5. Location
A box in a closed closet concentrates smell and makes some cats avoid it. A quiet spot with airflow is better for both of you. One box per cat plus one spare is the standard advice, and it holds up.
What doesn't work
Scented sprays over a dirty box, air fresheners next to the box (cats often avoid them), and liners, which tear and trap urine underneath. Save the money for a better box and a better scoop.