K.B.
What I used for my tripets who were three different shapes and sizes was the one from Baby Bjorn. It runs about $20-25 online. What's great about it is that it's oval shape! She won't fall through, no spray or splatter mess, rubber grip bottom. The splash guard is also a handle. The rim of the pot is very wide for less mess and leaking through. It has a high back for comfort and longer sitting time if needed. These worked out fantastic! If you need something a bit less expensive just look for the oval shape locally through another brand.
K. B
mom to 5 including triplets