Stiff hips turn a normal tray wall into a small climbing problem. A cat who once hopped in without thinking now has to lift each hind leg high, balance while she does it, and land on shifting litter, and many seniors quietly solve that problem by going beside the tray instead of in it.
One Low Side, Higher Sides Elsewhere
Choose a tray where one entry side is about four centimetres high, low enough for her to walk in with a near-normal stride rather than a jump or a scramble. Purpose-made senior trays are shaped this way, and a sturdy storage box with a doorway cut and smoothed into one side can serve the same shape if the plastic edges are left blunt and safe. The other sides should stay higher, because a cat who no longer squats deeply tends to urinate higher up the wall, and she still digs with enthusiasm even when her hips complain. Low everywhere means litter across the floor; low only at the entrance means easy access with containment kept.
Check the step from both directions. Getting in is only half the trip: she also has to climb out with full paws, often at a slower pace, and an entry that works one way usually works the other because the height is the same. If she pauses, rocks back, or puts a paw up and withdraws it, the side is still too high for her, whatever the packaging promised.
Room to Turn Without Twisting
Hips that are stiff do not rotate well, so the inside of the tray matters as much as the entrance. She needs a footprint long and wide enough to walk in, turn in a gentle arc, dig, and settle facing a comfortable direction, instead of shuffling round in a tight circle with her hindquarters pressed against the wall. If she regularly hangs her back end over the edge or misses from inside, the tray is probably too small for the turn she can manage now, even if it fitted her younger body well.
Place the tray on a non-slip surface, or on a low-pile mat that does not slide, because a tray that skids as she steps in teaches her that the entrance is unsafe. Keep it somewhere quiet and reachable on the floor where she spends her time; a perfect tray at the top of stairs she now avoids is no tray at all, and a second tray on another floor can be the difference between comfort and an accident she is embarrassed by.
When the Tray Is Not the Whole Answer
Give a new tray a fair trial in the right place before judging it, and keep her old tray available during the change so she always has a toilet she trusts. Watch for the pattern of any misses: beside the entrance suggests the step still defeats her, across the room suggests the journey is the problem, and inside but over the edge suggests turning room. Each pattern points to a different fix, and guessing wastes money on trays that solve the wrong half of the problem.
A cat who cries at the tray, strains, or stops using every tray you offer needs a vet rather than another design, because pain and urinary problems both wear the disguise of a tray preference. Tell the vet what you changed and what you observed, since the sequence helps separate a mechanical problem from a medical one. Recording entry height inside Pet Passport through KinsLe means the packet handed to a boarder or vet carries the tray setup that protects her hips.