Yeah kinda similar to what
@nerdnic did except I believe he used a plastic trash can. Sad thing is, I've only used one of them...it's on my Simple Cub and it's held up fine. Oh yeah, I could cut em out with scissors. At the standard firewall size they flex just a little so with that in mind, I would think it'd take a pretty hard hit to break it.
The only thing I would worry about is if the motor runs hot if it'd soften or melt the plastic which is something that 3D printed firewalls can face too.