the method i ended up using was a combination of posts. i lucked out and found a 23-7/8" inch particle board disk at the lumber yard (i guess its for table tops and the like) that i used as a pattern. saved me a bunch of work, and only cost $4.50. I used the disk as a pattern and drew circles on each 24" x 24" square piece of aluminum.

i rough-cut the circles using my little jigsaw and a fine metal cutting blade. That took a long time, but it got done. I tried to keep the cut to about 1/8" - 1/16" outside the circle i'd traced.

Then i clamped the aluminum disks to the wood pattern disk and did the final cut with a router. I used a 1" long "straight" or "edge" cutter and cut four pieces of aluminum at a time. My router slows down to 15000 rpm, which was a little fast, but worked well enough. the first pass was a bit slow and left some roughness, but a very quick second pass cleaned it all right up. The router bit has a small bearing wheel that followed the outline of the wood pattern perfectly. The final cut was so smooth i didn't need to do any edge sanding or de-burring of the pieces.

i've never used a router before (!) but i didn't have any trouble at all. rough-cutting the disks first eliminated most of the aluminum, so the routing went easily. It would have been MUCH faster to use a bandsaw for the rough cut, but i don't own one. if i were going to do it again i'd consider getting one. right now i have "excess capacity" (ie, more time than work) so it made sense to use what was at hand. the same procedure would work very well for irregular shapes. If i had a complex shape to trace, i'd consider having the pattern cut in plastic or masonite by somebody with a CNC router and then cutting the aluminum myself.

Total project time, including set-up and clean-up was 8 hours, or 15 minutes each piece. Depending on what they charge you, ts probably more economical to have the work done by somebody with a CNC router, unless you have more time than money, or if you're in a hurry (since there would also be the cost of transporting the material and waiting for the other shop to do the work).

And yes, there are little aluminum chips everwhere! Now i've got a 25 pound box of scrap aluminum to take to the recyclers.

Cutting a decent circle is similar accomplishment.

Thank you for taking the time to fill us in on the conclusion of your adventure.