Which bank account should I open in Singapore that allows me to store IDR currency?

INDR is not a major currency and the banks (OCBC, DBS, Stan Chart, HSBC, UOB, CIMB, Maybank) do not accept it under their multi currency account. However, if your INDR is from a small business, you can open a SME MCA account with DBS.

Alternatively, you can convert it to other major currencies like USD/EUR or SGD for foreign/local deposit. Transferwise allows for Indonesian rupiah account in Singapore, but do not accept physical curencies.