Indonesia is just as "made up" as any other multi-ethnic non-traditional ex-colonial statehood, like India for example. And frankly, the nature of its nationalism is quite similar to US in that it's inspired by revolutionary spirit and lofty ideals, which is why our jingoist tantrums can be as annoying as Americans.
Back to topic, the key here is to make Indonesia a more successful economy. Which is by PREVENTING '50s parliamentary period, assuming Indonesia achieving independence the OTL way. The decade-long warring period starting from Japanese occupation followed by war with the Dutch devastated its economy, polarizing and radicalizing Indonesian politics, also militarizing the entire country into effective warlordism, and such could've only be repaired by a steady government enforcing consistent policies from the start, while multi-party parliamentary system can only result in the disasters OTL has witnessed. Then, you can have it liberalize with time into a stable democracy.