I use the Stillen plate holder, plus a extension. The shaft was extended from the front where the plate attaches. It's a common thread size and is easy to link with a short piece of threaded rod. As I said in an identical thread started last year, the machinst charged me only $15 to fab the extension.

The Stillen holder is not made of aluminum. I wonder if a support plate made of aluminum would flex at highway speeds.

A steel plate will flex as well, it all depends on the thickness of the steel/aluminum plate.

I too got a ticket for not having a front license plate and did not want to drill into the bumper. Here is my low cost solution. I chose to use industrial strength velcro to attach the front license plate and cover. The plate can be removed easily and the velcro comes right off the bumper as well.