Product review (5 stars): Best roof cap for 6" duct.
I am a contractor and I use these for every project that needs a 6" range hood cap on the roof since they are not commonly stocked at the Blue Box, Orange Box, or Green Box.

They sometimes come with the corners bent a bit, but that is perfectly normal.  The metal is soft enough that it can be formed by hand so taking the bend out is not a problem.  The damper and bird screen may need a tweak before installation, but that only takes a second. Check it over before you install it and it is not a problem at all.