You start out with a triangular piece of 0.25" inch material that has about 1/16" of excess material all around. I used a ??? drill (clearance size for a 10-32 screw). Bigger...

The holes are supposed to be countersunk so that the countersink just reaches the bottom side of the aluminum. After getting the countersink to just start touching the aluminum, you need to plunge by the thickness of the material. I tried a 5 flute countersink (typical for woodworking) and it didn't work very well. I found the single flute countersink worked much better (you're removing 1/5 of the amount of material). Bigger...

Here's a picture of what your drill chips will look like if you get the feed and speed correct. Once the chips get to be a bit longer than the length of the drill bit, you should stop advancing the bit so that the chip will be broken, and you can put a bit more cutting fluid in the hole. With the chips like this, you don't need to back out the drill bit to clear the chips. In this picture I let the chips get a bit longer, but you need to be careful because these are whipping around at the same speed as the drill bit and they're sharp! Bigger...

You also need to drill through the scrap as well. Here you can see the dimples left from drilling through the main piece. Bigger...

After drilling the holes through the scrap, I realized that the scrap was getting hung up on the T-Nuts. Here you can see the top of the T-Nut protruding above the T-Slot. I had to go back and counter drill the bottom side of the scrap to allow for the T-Nut clearance. Bigger...