Imposing an additional filter LC will impose additional
series resistance and degrade load regulation. Maybe
this is tolerable, or not.
More shunt C from output to GND will further suppress
the ripple, but you want to have a good look at the
character of the ripple - baseband, or HF? - to pick
what type. And beware that the converter loop has
been compensated based on the present, plus perhaps
some range of added C and too much could be destabilizing
depending on the control loop design.
Offhand I'd say you might get to your goal by adding 2X
the present output filter C, of similar ESR/ESL, in shunt.
Or, you could get a +/- 16-18V setup with LDOs to clean
it up further. If your current requirement is modest at
least.