Small chance a rivnut and a button head cap screw might secure things- time to pull the crankset anyway and clean it up.
i see fitting, hacksaw and files needed to accomplish if the correct part cannot be found.
possibly a T nut too but spot facing the back to get the base flush will probably be required.

small obsolete parts that were never easy to find are a problem. A TA chainring fastener for the 6 point intermediate ring might work too... another easier to find part but expensive as a set.