I also have played Ovations that kept slipping while sitting.
I have a 1984 Ovation Custom Legend 12 deep bowl. Besides being very deep it has a flatter spot on the bottom with a rough gripping surface that prevents this. After 26 years it still has not worn off at all.
I did have a shallow bowl 6 string once that was impossible for me to play sitting. It was made in the 90s and had no gripping surface on the bottom. After cleaning the guitar, it was like trying to hold on to a bar of soap. It made me crazy.
Wayne,