Sorry to hear about that. My parents lost two of my childhood cats to coyotes when they moved to the Pacific NW, so I know how that feels. My deepest, deepest sympathies.
My only recommendation is to keep them indoors. Having to change the litterbox every day (or at least every few days) is a small price to pay.
My only recommendation is to keep them indoors. Having to change the litterbox every day (or at least every few days) is a small price to pay.