Routers with a flush cut straight bit work really well to trim Alu to size, for mine i 3d printed an actual routing template as i had sloped sides on the insert section so two different circle sizes.
Not an issue for what you want to do if you can fit the print onto the bed.
So rough cut with jigsaw and trim to size with the router.... umm Alu chips are bloody hot when they fly off
though a quick ebay search turned up things like
Wouldn't of even thought to check eBay for pre-made ones, that'll save some bloody time! They come in varying sizes as well.