OK, took a couple of reads to get what you were talking about.
I would call the aluminum a pattern. I use mainly 1/2" plywood for almost everything. So I would trace the aluminum pattern onto a piece of plywood. Then like the others said, cut out what you don't want, leaving preferably a bit less than 1/8". With straight lines like that I would normally tack down a straight strip, route one are, move the straight strip. I use nails because they are easy to use, no cleanup, and can reuse. And they work. Can cut the corners or sand square. Once I get that piece done exactly the way I want it, I trace it onto another piece of plywood, rough cut that piece, then glue the finished piece to it. Then I rout that, which gives me a master 1" thick. I normally make masters because there are few things I do not make more than one of. I drill pilot holes thru, trace around it onto my finish piece. Be sure to tack the master to the back of your finish piece, so no holes will be visible. Then you can square the corners. If you want more copies, repeat.
I do it a bit different that the other guys, do use a flush trim bit, but I also tend to make different things than the others. Works for me. Oh yes, I also put a drop or three of Marvel Mystery Oil on my bearing before I start, don't try to hog with my bit, and don't tray to rout too fast. Works for me.
"It ain't what you're told, it's what you know." - Granny Weatherwax
Some days, the supply of available curse words is insufficient to meet my demands.
Call me a craftsman, artisan, or artistic, and I will accept that. Call me an artist and you will likely get a quite rude comment in return. I am not a @#$%ing artist.