Originally Posted by
oneclick
I suggested drill because (assuming he gets the correct size) the walls of the hole should be square to the chainring. While this is not guaranteed with a drill[1], it's guaranteed NOT with a tapered reamer. One end of the hole (or both, if you go from both sides) will be too big when the other (or middle) just fits; and considering the application and material such a mount will wear quickly.
As well, using such a reamer takes some skill. It's all to easy to make a hole that is not round at all, but a multiply-lobed hole, one lobe for each of the reamer edges, and of course, it is a lot easier to make the hole too big.
[1] Holes made by drills are not guaranteed to be any of the following:
a) round
b) straight
c) the same shape all the way through
d) the size you want them to be
e) where you want them to be
Well, sure, I cannot disagree. If OP has a drill press and a 10 mm bit, then that is the way to go. With that in mind, ideally the ring should have a 10 mm hole, enlarged to 12 mm about halfway through.
If he's working with hand tools, though, I stand by the reamer suggestion (a rat tail file is also an option, but requires a bit more skill). In practice it is pretty easy to enlarge a hole in aluminum, a little bit at a time, trying the bolt repeatedly, until the bolt will go through. it should be a tight fit.