I just did one for a winch opening. I would mock it in cardboard and get it perfect. Get a piece of aluminum and use a metal blade in a jigsaw. Sand the burrs off the edges. Give it a few coats of bumper black and you’ll be in business. Secure it with what looks like 2 machine screws and nuts.
Like this. But much smaller. And for a bumper end cap…
As suggested make a cardboard template then transfer it to aluminum and cut with a jigsaw, I made some plates for aux backup lights using this method and .125 aluminum very easy to work with.