center diff mount

[Urgeoner]

I just started to make mine, shamelessly copying Lonewolf's design. Cut out of 3/8" aluminum, its pretty thick...I just used my brothers bandsaw and my handheld drill. I want to drill the hole for the center diff bearings but all the hardware stores sell the 5/8" bits at like $18 a piece and thats a bit too much for me atm for a bit I will hardly ever use.. I saw a step bit at one store but it wont go deep into the thick metal..
Any thoughts?