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Great job!
To open the hole up a little I would suggest a rat-tail file. Aluminum is surprisingly easy to work with.
Going at it with a larger spade bit will be a mess, as you won't have the pilot spike to keep it from slipping. Hole drills will have a similar problem.
An alternative is using a large twist drill (an end-mill would be much better) in a hand drill and try and cut the edges down with the side of the bit. This gives sloppy results, and is kind of dangerous, but could get the job done. Some twist drills are made better for this than others, so if you go this rout; try all the larger bits of different makes you have access to until you find something that works (but you should really just use a file).
This seem like one of those projects that would take 1/20 of the time with a milling machine and proper tooling. It's always more frustrating knowing that there is a better way, which you don't have access to, but such is life.
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