I did the deed last weekend and it went very smoothly!
Just wanted to post to put others at ease.
The day was about 75 degrees with sunshine shining in the hanger.
Used a 3" cutoff wheel in a die grinder.
It cuts like butter! Piece of cake! Worried too much for sure!
So if you haven't done yours yet, don't let it scare you. Easy peazy! Course sucess is due to all the help from this site!
Oh, I did leave a few un-cut tabs then cut those off after I had a couple extra sets of hands to hold it all still and in place so I wouldn't put any stress anywhere.
No, I did not warm up the hanger other than the sun shining in through the open door. I thought about putting a dark colered item under the bubble so the sun would warm that up and heat the inside, but didn't. Apparently the warmer the better though.