Hi bhumatt,
The first post of this thread has some foundation options that can sit on top of the slats. I don’t know of any that are 12" high but there are several that are in the range of 8" - 9".
If you needed more height you could always add a very firm polyfoam layer underneath the foundation (the firmer the better) or even a 2" to 3" bunkie board although two foundations on top of each other may have a tendency to shift with movement on the mattress.
Phoenix