Iso DC-DC PCB heat dissipation technique proposal
The casing maximum temperature is 105 C. (very hot)
At 90% worst case efficiency and full load it only loses 4 Watts.
At 60C Ta you can still do maximum load since with 10.8 C/W it's at 103.2 degrees celsius.
You can create a thermal pad with a via-stitching grid to all layers under the module and put some thermal transfer pads on it. Maybe transfer the heat to a plate on the bottom mounted a similar way. Or directly to the enclosure with an aluminum spacer.
Test it! Solder some wires on the thing and grab some copper clad fr4. Cut the fr4 to strip and put it on with some thermal adhesive pads.

But the difference between no heatsink and heatsink is only 0.5 K/W, so I doubt any case mounting options will be very effective.
If you need it to run 100% load, you're going to have to limit your ambient temperature to below 60 C. Or get a different series, eg: TEP 75WI.