This is a good point. That said, I sort of suspect that natural convection is enough in the X1C already, given that the heat sink is coaxial with the original fan (and, therefore, the fan is presumably not doing a whole lot to move air over the heat sink…). Maybe one of these days I should stick a thermocouple on that heat sink and see how necessary it really is.
I’ve updated my Thingiverse description to suggest PETG. Thanks for the suggestion!