It depends on how you want to use the cowbell. I usually attach it to the top of the hihat pin or the ride cymbal, when I use it as a substitute for the ride, hitting it on every quarter/half note, for instance. Many drummers use this location for various percussion instruments such as tambourines.

Woodblocks follow this pattern: they are often used where a snare drum would otherwise be used, so people attach the woodblock to their snare drums. By doing this, you don't need to change your position behind the drum kit to adapt for the change in your setup.

(If you have an large rack of percussion however, this answer will probably not be very useful to you.)

It depends on how you want to use the cowbell. I usually attach it to the top of the hihat pin or the ride cymbal, when I use it as a substitute for the ride, hitting it on every quarter/half note, for instance. Many drummers use this location for various percussion instruments such as tambourines.

Woodblocks follow this pattern: they are often used where a snare drum would otherwise be used, so people attach the woodblock to their snare drums. By doing this, you don't need to change your position behind the drum kit to adapt for the change in your setup.

(If you have an large array of percussion instruments however, this answer will probably not be very useful to you. It's not always possible to attach everything to some part of your drum kit, in which case you'll either have to kill your darlings, or use a separate percussion rack.)