The best solution "to avoid being noticed" is to have a stock
cover in place. I have carved a few bumper covers for people so
they fit around their mods.

What's needed to be strictly legal is going to be discussed by lawyers,
but generally, if it "looks right" to the cops, the cops won't bother you.
If you keep plenty of following distance, then you won't crash. If you
don't crash, probably no one will notice or care.

Some trucks have a piece of pipe for a bumper. One of my former autocross
cars had a piece of aluminum tubing that was a "bumper". Plenty of rods and
customs have a push bar and/or a "custom" bumper. -Bob