Safety 1st Tubside Bath Seat

My 6 month old son doesn't really fit in the seat well (very chubby legs) and the arm rest doesn't fit over the edge of our older tub - it is too thick. Newer tubs would probably be ok. Without being able to hook onto the tub, the seat is too unstable. BUT if you could actually put it in the tub like it should be used, I think it would be a great helper for those babies too big for an infant tub, but can't sit yet. The fact that the seat swivels is wonderful.