Need some litter box ideas

I finally got a litter box that Lucy can't pee over the side (she will stand on her tip-toes to do it) but now I need ideas to help Carly. We had to cut the hole in the side because Much is senior and isn't going to be able to jump over the sides. Carly's problem is that her tail doesn't work so she will sit right at the entrance so it holds her tail up but right now she is peeing on the outside of the box. This means that I have to clean the area several times a week. Does anyone have any ideas of what I can put inside the entrance so she will move farther into the box?

Here's a picture of the box right now.

I hate to ask a really stupid question - but will your kitties use a covered box? I've got a perfect box in mind to meet all the needs - but it's covered. If they won't use a covered box, maybe there's some way to creatively make the same type of thing with the really tall-sided boxes you've got. ??

Here's a link to the box I'm thinking of: it's called a Booda Dome Cleanstep Litter Box. It's not the small Booda box - it's large, round, and has stairs going up one side, and an oval shaped space on the other side (next to the stairs) - so kitties have to walk up and then into the box to use it. Because it's got the ramp, your senior can use it. Because it's covered, Lucy won't pee over the side, and because you have to go up and into the box, Carly would at worst pee over the side, it runs down the stairs and collects at the little hump at the bottom - easy to clean with a paper towel.

http://www.catsplay.com/bci_683805.php3

Good luck!

Laurie