A Flex PCB connecting the OCP2 connector would allow to put the converter board behind the NIC board, allowing the NIC board to be exposed to the aluminum case to use the case itself as a heatsink (would need a split case so the NIC board can be screwed to one side of the case, pressing the main chip against it via a thermal pad).
As a stop-gap, I'd see if there was any way to get airflow into the case - I'd expect even a tiny fan would do much more than those two large heatsinks stuck onto the case (since the case itself has no thermal connection to the chip heatsink).
As a stop-gap, I'd see if there was any way to get airflow into the case - I'd expect even a tiny fan would do much more than those two large heatsinks stuck onto the case (since the case itself has no thermal connection to the chip heatsink).