Product review (1 stars): Hardly fit. Did NOT keep squirrel out
Sorry but the squirrel figured it out in a day. We had a professional roofer  install three of these. He said once they were installed the inner lid wouldn't close tight but it was close enough.
Basically you really need to measure your opening. Mine was 20 feet off the ground so we couldn't . The roofer was kind enough to put it one for me for only $150.00. Money wasted indeed.
I can hear that $@&&$&'squirrel gnawing away at my rafters as I type.
Now we have to have the squirrel removal people come and put traps on the vents to catch the varmit . I hope there is only one . They said most likely there are a couple. They charge by the squirrel. This will only cost $ 1200 to start and then a fee per squirrel.
I swear, I am close to removing the sheet rock in the ceiling and putting the trap I bought up there.
Anyway, believe the reviews that say they don't work.