I agree with isherwood but I have dealt with cities that want an extended box (6") with a sealed opening. You are especially going to notice this in cities that require an inspection before selling the home.
So the answer is you need to call up your inspector and just ask them what they want. If the project is big enough they may ask you to pull a permit. I know I hate pulling permits for cosmetic stuff but when you poke holes in a firewall in garage for new plumbing for whatever the city will put 2 and 2 together and may make it a very long inspection... when they could just want some fire rated foam.
Note: that when you call the inspector and he says "Just do this". You can disclose this conversation to future inspectors and to potential buyers. Just having record of conversations like this have saved me days of work. New inspector comes out and whatever you did is grandfathered - instead of you doing whatever he wants.
To do this by strict firewall code I found a kit/brace that would meet standards. 3M makes it so I am sure it is available for schedule 80 and available in the US. Link to kit. (if someone finds US link add it)