While you can cut aluminum with ordinary woodworking tools, it makes a hecka mess and the chips are nasty sharp. And by the time you’ve got a big enough, thick enough piece, you aren’t that far off a store bought plate.
A phenolic plate from amazon is only $26 and comes with hardware https://www.amazon.com/DCT-Universal-Router-Insert-Inches/dp/B077383KDP/
If you are going to mostly leave it table mounted and don’t need any fancy stuff, you can just screw it to a piece of 1/2 plywood. There are many videos on this on YouTube.