2008 xl heater

[Doubeleive]

Doubeleive, I found one for sale like the picture I posted here and it appears to be like your right picture but with the long cooling (?) duct missing. Does that make sense?
It is the part number that GM parts sites list for mine but mine has the short duct like your left picture.
Would the right one on your pictures would properly without the cooling dict?

You might be able to transfer the cooling duct over from your old one not sure

 

[EddieC]

You might be able to transfer the cooling duct over from your old one not sure

I tracked down the current part number for my Tahoe on both GM parts sites and Rock Auto and RA had the clue to why there is a hole. There was a link to recommended auxiliary part; a replaceable long cooling tube like you showed on one of your pictures. The issue could be that my original fan has the integral short cooling tube, so, as you mentioned, clearances could be an issue with the long tube.