How to Install Roof Vent Guard?

I need to know the way to install a roof vent guard over a round roof vent fan.
The round roof vent fan has a screen covering the air outlet. But the screen has no frame to stiffen it. Consequently, squirrels push the screen away easily. They lived inside the roof for some time until I finially get rid of them. Now, I am sure that no squirrel is inside my roof any more. I need to keep any new comer away.
I want to install a square steel cage over the round roof vent fan. The problem is that I cannot find any instruction about installing it anywhere in internet (they only say that it is easy to install). Therefore, I am not sure if this is something that I can do. I don't want to nail it in place because I am afraid that I may add a way for water to leak inside the roof.
One thing that I want to point out is that I have a very low pitch roof. The roof doesn't have any shingle. It is covered with a thick layer of roll-roofing. I am not sure if this makes any difference or not. I want to mention about this in case this makes any difference.
Please tell me the appropriate way to install a steel roof vent guard. Thanks in advance.
Jay Chan
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