What type of aluminum do you have is it sheetmetal or just solid and how thick. Looks like a dremel will be the solution.You'll need a drill bit cutting discs a file and patience.
Yeah,a dremel is a good way. I marked out my slots with a sharpie first and colored it in. I then used a 1/8" drill bit and made a series of holes. Then I used the dremel with a cutting disc to take out more material. Then I used a mill file to fine tune them. And finally a set of micro files to make them "perfect".