If it is smelling, it needs cleaning out more often - spot cleaning daily is a minimum. I find that my outdoor rabbits need the trays completely emptying and disinfecting twice a week, as well as daily spot cleaning. Using a thinner absorbent layer and changing it more frequently works out cheaper, and better on the odour front. I buy the biggest / cheapest bags of paper based litter I can find (online or at B&M stores). Newspaper also makes it easier to empty the trays. Hay on top keeps it all in the tray and gives the rabbits a reason to use them.