The placement is mostly determined by the design of the OCP 2.0 connector. OCP 3.0 has a connector at the short edge of the card, which allows exposing/extending the heat sink directly to the outer case.
If somebody has the talent, designing a Thunderbolt 5 adapter for OCP 3.0 cards could be a worthwhile project.
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).
If that's not a requirement just get the Raiden Digit Light One, which does have a fan (and otherwise the same network card).
If I could design an adapter PCB myself, I would go straight to OCP 3.0, which allows for a much simpler construction, and TB5 speeds.
Alternatively, there are DELL CX422A rNDC cards (R887V) that appear to have an OCP 2.0 connector but a better heatsink design.
If truly concerned, one could use SFP28 to SFP28 cage adapters to have the heat outside the case, and slap on some extra heatsinks there.