I'd probably go for a little widget to clamp into the axle slot of the dropout, with a drill bushing pressed into it. You'd then be drilling into a concave surface, which is less likely to push the drill off centre, and you're using the most regular features of the existing component as your alignment guide.
I'd probably go for a little widget to clamp into the axle slot of the dropout, with a drill bushing pressed into it. You'd then be drilling into a concave surface, which is less likely to push the drill off centre, and you're using the most regular features of the existing component as your alignment guide.