Hi NatalieC,
You can see my thoughts about a non breathable surface between the mattress and foundation in post #10 here and I personally would avoid it unless there was a compelling reason not to and the other risk factors were low.
Besides just adding more slats … a thin (1" - 2") layer of very firm polyfoam or something like the bed rug here or one of the slat conversions here (which has no flex at all) or even one of the Ikea slatted bed bases here (which has some flex which may change how the mattress feels and performs) would work well to even out the support and prevent the latex from sinking through wider gaps and would have more airflow and ventilation.
Phoenix