Updated fan - need advice on how best to cut plastic

[Jorgen]

Hello, I updated the bathroom vent this weekend - pictures can be found here The new plastic collar at the interior needs to be cut down to fit. I don't want to break it and have been considering using a sharp Olfa knife. What/how do folks recommend cutting the plastic collar?

[Bassdogs]

As I remember when I installed 2 of these, I used a razer knife. You can also use a cutoff wheel on a Dremel. These shrouds are made to be cut down so it is unlikely you would break it.

[Aardvarkjff]

I use a multi tool and a fine tooth blade (usually for wood & metal). I like the half moon shape. Then I just trim off the rough edges and good to go.

An angle grinder with a cutoff blade will work too.