Most hole saws will cut Alum.some are made to cut mild steel,the same thing is true for drill bits..
The main hole,,,You only want to do this one time, so it should be a 3 3/4" hole to take on the big panel bits,,then you will run into a error because a 3 3/4" / 4" hole saw is not cheap and then you need a lip to hold the insert ring...on top of that, you can use a fly cutter in the drill press that will do a good job..and put in the main hole and the lip. http://www.grizzly.com/catalog/2007/Main/614
But if you say I don't want to go that big at this time, that's fine, just chuck up the 2 1/2" hole saw and start the hole in the drill press but only drill 1/2 way into the plate,remove the plate from the drill press and use the 1/4" hole to line up the router on the Alum. plate,,a 1/4" dowel pin works great for this job,chuck up the pin and use router to line it up,draw around the base plate and remove the sub base plate (the black one the norm) now mark the holes to mount the router to the new plate,then drill them out and counter sink them once you done that test the fit,once it fits then back to the drill press and drill out the 2 1/2" hole,,,,now for the insert rings, start with one that is 3" OD and 2 1/4" ID once you have it made 3M glue/super glue, it to the base plate this will give you the lip to hold the rings...then make the other rings the same way but they will all be 2 1/2" OD to fit on the plate...you will need to use the same stock for the rings they need to sit FLAT in the hole..
Hope this helps