The chassis opening is smaller than the IR lens but it still works. If the lens were more exposed it would probably have a better off-axis performance. As it is, there’s at least a 45 degree off-axis range in either direction from on-axis.
Something like that fan header would probably work fine. IIRC, I soldered to the board in place of the IR receiver a 3-pin header with 100 mil spacing. I also made a cable using some spare 5-pin 100mil housing (such as that you would use for an internal USB connection or similar).
