This fan duct allows for using a 40mm axial fans, which are easier to find than 30mm fans and seem to last longer anyway.

If you're using a 40mm fan to improve cooling, be sure to also use computer thermal paste on the threads of the heat break's upper end. That will probably help at least as much as this fan duct does.

The fan duct clips onto the barrel without any extra hardware. For me, it seems to stay put, stays aligned and doesn't rattle, and easily pops off with gentle but firm force.

The intake is optional, but I find this eliminates fan wash on the intake side. I haven't done any tests to see if this improves air flow, but I've found fan wash to interfere with part builds using ABS. I've made 10mm deep intake ducts before (also on Thingi) but this is 8mm to improve clearance. It seems to work just as well.

I'm including solid model files for easier modification by others.