This is a Digium card, clearly intended for a 1U or ATX case. One of my goals is to reduce the number of high speed fans in the lab, so I repurposed my Lanner chassis. Using a typical twist drill bit is a poor choice for the job of an end mill, but it came out ok when I put a rotary steel brush to the aluminum plate.
This was my brain wave today. I need to remember to do this the next time I want to make a bracket.