I would do the drawing as follows:

• Go to random.org and generate the random number(s) between 0 and n based on a "persistent identifier". A good identifier to use would be a newly generated bitcoin address that has not been used yet.
  
• Post the SHA-256 hash of the identifier on the forum to commit to the number(s)
• Publish the guesses. This way, people can see if all guesses have been exhausted. This prevents me from claiming no-one won.
• Announce the winner and publish the identifier used to generate the number(s), so everyone can verify the winning number(s).