I bought an extra castle nut and used it as a drill guide. Drill one side, pin it with a shortened cotter pin, then drill the other side. Sharp bits are a must!
The shortest drill bit I could find in a small head 90 degree drill... oil and go, did both sides in about 5 minutes. I think the overall bit length was 5/8" .
If you have Dremel tool it's a good idea to flatten the spot before using the drill. Carefully mark the hole location then use Dremel to shave part of the threads where drill will be going and you are all set with precision work. High speed drill, sharp bit, lubricate a lot and in 10 minutes as Mike said you will have nice matched holes for your cotter pins.