[redshift]

Wow glad you survived all that :)

Since you're cutting aluminum, tool steel is your friend... time to play on the old grinding wheel.

First tip, use an aluminum oxide wheel for grinding tool steel aka HSS.

Silicon carbide wheels are normally only used for cemented carbide bits, and super-hard materials.

Form your relief so the chip material 'flows' as shown on the end view of my super-professional illustration, see tool bit on left of photo. The actual cutting width can be far less than the space, you'll go in a little, then pull the cutter away from the chuck, repeat til you have your proper ID & width. I usually use a dial indicator to know the depth of cut in a counter-bored hole.

The hardest part is measuring the bore inside the 'lip', if you have a junk pair of calipers, carefully grind away a relief on the inner measurement end of the caliper jaws, to look much like the cutter you used, so the measuring tips fall inside the lip, make sense?

Tool steel is cheap and you may have a few chunks lying around already, you'll need no more than 1/4" or 3/8" HSS blanks.

Use a silicon carbide stick to dress the wheel to the profile you need.

Standard wheels are 3/4" wide, so you'll want to cut a step in the wheel for grinding narrower gaps in your HSS bit. No need for fancy tooling here...