Old cat struggling with litter box

Hi!
I have a 16 year old cat who's overall really healthy, just some minor arthritis. I think due to her age and being unable to squat all the way down, she's having a hard time getting all the way into the litter boxes we have. We've tried larger litter boxes and taller ones, she always makes it into the box, but she manages to hang her butt over the side and wee on the floor, we've resulted to putting puppy pee pads under all the litter boxes. We've spoken to her vet and the vet agrees it's just due to her age and no other serious issues/illnesses, so we're looking for advice on how we can get her to get all the way into her litter box and stop weeing on the floors, we're exhausted! We'll do anything we need to to make her as comfortable and get her as easy access to her litter as possible, we just don't know what else to do.

Any advice is greatly appreciated, thank you!